cosmides_1985_chap5

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Chapter 5Human S o c i a l Exchange

E v o l ut i o n ar y b io l og y p r o v id e s a h e u r i s t i c f o r g u i d i n gp s y c h o l o g i c al t h e o r y ; t h e r e s e a r c h and t h e o r y p r es e n t e d i n t h i sd i s s e r t a t i o n i s meant t~ be an i l l u s t r a t i o n of i t s p o t e n t i a l .T h is h e u r i s t i c r e s t s on t h e r e c og n it i on t h a t n a t u r a l s e l e c t i o nha s p roduced psycho log ica l mechanisms a s response s t o va r i ousse le c t io n p res sure s . The more impor tan t th e ada p t i ve p roblem,t h e more i n t e n s e l y s e l e c t i o n w i l l have sp ec ia l i ze d and improvedt h e perform ance of th e s e mechanisms. Some of th e s e mechanismsevolved t o mee t t he ada p t i ve prob lem of s oc ia l exchange.S u c c e s s f u l l y c o nd u ct e d s o c i a l ex ch an ge was a c r i t i c a l l y im p o rt a n tf e a t u r e of ho minid e v o lu t i o n . N a tu r a l s e l e c t i o n p e r m i t s t h ee v o lu t i on of o nl y c e r t a i n s t r a t e g i e s f o r e ng ag in g s o c i a lexchange. By s tud y ing th e na t u re of the se s t ra te g i es , one candeduce many pro pe r t i e s t h a t human a lgor i t hms regu la t i ng so c i a lexchange mu s t h av e, a s w e l l a s much a b o u t t h e a s s o c i a t e dc a p a b i l i t i e s su ch a l g o r i t h m s r e q u i r e t o f u n c t i o n p r o pe r l y . U sin gt h i s framework, one can then make empi r ica l p re d i c t io ns abou thuman p er fo rm an ce i n a r e a s t h a t a r e t h e t r a d i t i o n a l c o nc e rn ofc o g n i t i v e p s y c h o lo g i s t s : a t t e n t i o n , c om mu nica ti on , r e a s on in g , t h eor ga ni za t i on of memory, and lea rni ng. One can a l s o make s p e c i f i cpr ed ic t i on s abou t human peformance on reason ing t e s t s l i k e theWason se le c t io n ta sk .

Chapter 5 e xa mine s t h e n a tu r e of t h e s e l e c t i v e p r e s s u r e s o ns o c i a l exchange i n human evol ut i on, and what th es e a l low one t oi n e r a b ou t t h e p s y ch o l o gi c a l b a s i s f o r s o c i a l e xc ha ng e i n


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humans. I t i s d iv id e d i n to t h r e e p a r t s w hich make t h e f o l l o w in gp o i n t s :5.1: Only c e r t a i n s t r a t e g i e s f o r e n ga gi ng i n s o c i a l ex ch an ge ca ne vo lv e: n a t u r a l s e l e c t i o n ' s game t h e o r e t i c s t r u c t u r e d e f i n e s

what p r o p e r t i e s t h e s e s t r a t e g i e s must have.5 .2 : The e c o lo g i c a l c o n d i t i o n s n e c e s s a r y f o r t h e e v o lu t i o n .o fs o c i a l exchange were mani fes t d ur in g hominid evo l u t i on ;h om in id b e h a v io ra l , e c olo gy f u r t h e r c o n s t r a in s acomputat i onal theor$ of s o c i a l exchange.5.3: T he se s t r a t e g i c a n d e c o lo g i c a l c -C fn str aint s d e f i n e a s e t o finfo rmat ion pro ces s in g problems t h a t must be sol ve d by anyhuman engaging i n s o c i a l exchange. Computat ional th e or ie sof th es e p rob lems a r e deve loped. '

5.1 N a tu r a l s e l e c t i o n a nd s o c i a l e xc ha ng e:Only c e r t a i n s t r a t e g i e s f o r e ng ag in g i n s o c i a l e xc ha ng e c ane vo lv e: n a t u r a l s e l e c t i o n ' s game t h e o r e t i c s t r u c t u r ed e f i n e s what p r o p e r t i e s t h e s e s t r a t e g i e s must h av e.The c r i t i c a l a c t i n f or m ul a ti n g co mp ut at io na l t h e o r i e st u r n s o u t t o be t h e d i sc o v er y of v a l i d c o n s t r a i n t s ont h e way t he world i s s t r u c t u r e d . . .

-- Marr & Nish ihara , 1978

T he re a r e l a ws i n h e r e n t i n t h e dynam ics o f n a t u r a l s e l e c t i o nt h a t hold fo r any sp ec ie s , on any pl an et , a t any t ime. Many ofth e s e l aw s g o ve r n t h e e v o lu t i o n of s o c i a l b e h av ior ; t h e yc o n s t r a i n t h e k in d s of s o c i a l b e ha v io r t h a t c an ev o lv e .

T r a i t s c a n be t h ou g ht of a s t h e e mbodiment of ~ t r a t e a i e s o rth e propogat i on of th e genes t h a t code f o r them. By anal yzin gt h e dynamics of gene flow through po pul ati ons , one can determin ewhat ki n d s of t r a i t s w i l l quick ly be se le c t ed ou t , and wha t kindso f t r a i t s a r e l i k e l y t o become u n i v e r s a l and s p ec i e s - t y p i c a l.Formal ly , t h i s an a l ys i s can be ca s t i n t e rms o f game theory : one


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s t r a t e g y i s p i t t e d a g a i n s t a n o th e r i n a r a c e t o s e e w hich onecomes t o dominate th e gene pool . Such games can bemat hemat i ca l l y modeled w i t h g r e a t p r ec i s i o n , * D ur in g t h e l a s t 20yea r s , game- theo r e ti c model s o f t h e dynamics of n a t u r a l s e l e c t i o nh av e p r o l i f e r a t e d i n e v o l u t i o n a r y b io lo gy . T h i s p r o c es s h a s l e dt o a s t a r t l i n g d is co ve ry : U e r e u ertain s t r a t e a i e s thatsimwlv cannot Le s e l e c t e d fPT. (e.g. , Hamilton, 1964; Will iams ,

't

1966; Maynard Smith , 1978; Dawkins, 198 2) . Fur the rmo re, game-t h e o r e t i c a n a l y s e s ca n be u se d t o s p e c i f y what s t a t e g i e s a r el i k e l y t o be s e l e c t e d f o r , and what p r o p e r t ie s t h e s e s t r a t e g i e sm us t h av e. T h i s c l a i m d e s e r v e s a n i l l u s t r a t i o n f ro m t h el i t e r a t u r e of e v o l u t i o na r y b io lo g y.

G i ven an i nd i v i dua l , X , d e f i n e a BENEF I T TO X ( B ( X ) ) a sa ny a c t , e n t i t y o r s t a t e of a f f a i r s t h a t i n c r e a s e s t h e number o fr e p l i c a s of a gi v en g en e ( o f f s p r i n g ) which t h a t i n d i v i d u a lp r oduces t h r ough h i s o r he r own r ep r oduc t i on . S i m i l a r l y , de f i nea COST TO X ( C ( X ) ) a s any a c t , e n t i t y , o r s t a t e of a f f a i r s t h a tdec r eas es t h e number o f gene r ep l i ca s t h a t i nd i v i du a l pr oduces.J u s t t o e xh au st t he p o s s i b i l i t i e s , l e t 0 ( X ) r e f e r t o any a c t ,e n t i t y o r s t a t e of a f f a i r s t h a t h a s no e f f e c t on t h e number ofg e n e r e p l i c a s X pr oduces . By so de f i n i ng t h e e f f e c t s whi chd i f f e r e n t m or ph ol og ic al , p h y s i o l o g i c a l , o r b e h a v io r a l t r a i t s c anh a v e on g en e r e p l i c a t i o n th r ou g h a p a r t i c u l a r i n d i v i d u a l , one c ancompare t wo a l t e r n a t i v e t r a i t s t o s e e which o ne l e a d s t o g r e a t e r

* The Modern Synthesis -- t h e we dding of s t a t i s t i c a l methods t oMendel i an genet i cs -- b ro ug h t r i g o r t o e v o l u t i o n a r y b io l og y i n .t h e 1 9 3 0' s . D ur in g t h e l a s t 20 y e a r s t h a t r i g o r h as beens u b s t a n t i a l l y e n h a n c e d b y (1 ) t h e i d e n t i f i c a t i o n o f t h e g ene a st h e u n i t of s e l e c t i o n , and ( 2 ) t h e t e ch n ol o gi c al a b i l i t y t oc re a te computer models of s t r a t e g i c games.


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r e p l i ca t i on o f the genes which under l i e it and w i l l t h e r e f o r espread th rough t he popu la t ion .

Now, con sid er t h i s ex ce rp t from Hamilton, 1972:A gene i s b ei ng fa vo r ed i n n a t u r a l s e l e c t i o n i f t h eaggregate of i t s r e p l i c a s f or ms a n i n c r e a s i n g f r a c t i o nof th e to t a l gene pool . We a re go ing t o be conce rnedw i th genes supposed t o a f f e c t t he so c i a l behav ior oft h e i r b e a re r s, s o Let us t r y t o make t he argument morev i v i d by a t t r i b u t i n g ' t o t h e g e ne s , t e mp o r a r i l y,in te l l ig en ce and a c er t a in f reedom of choice . Imaginet h a t a g ene i s consi der in g th e problem of i ncr eas ing thenumber of i t s r e p l i c as and imagine th a t i t can choosebetween caus ing pure l y se l f - i n t er es te d behavior by i t sbearer A ( le adi ng t o more reproduct ion by A) and causing"d i s in te r es t ed" behav io r th a t ben e f i t s i n some way ar e l a t i v e , B. (p. 195)

Hamilton th en computes how many r e p l i c a s of t h i s gene w i l l beproduced if i t c o d e s f o r d e c i s i o n r u l e 1 v e r su s d e c i s i o n r u l e 2:

For any act , Z , which would benefit A ' s r e l a t i v e , B ( Z = B ( B ) ) :Decision Rule 1. I f [ C ( A ) of doing Z 1 > 0, do not do 2 .Decision Rule 2 . I f [ C ( A ) of doing Z ] < [ B(B) of r ece iv ing Z ]d i s c m t e d & r (A ,B ) ( a f r a c t i o n d e no t in g t h ep r o b a b i l i t y t h a t B c o n t a i n s a r e p l i c a of t h egene i n q u e s t i o n ) , then do Z .More re p l i c as o f the gene i n ques t ion w i l l be produced i f tha tg ene c od es f o r d e c i s i o n r u l e 2* r a t h e r t h an d e c i s i o n r u l e 1. T h i sr e s u l t h o l d s f o r s p e c i e s which c an s e l e c t i v e l y c o n fe rb e n e f i t s on r e l a t i v e s . I t i s a law inher ent i n th e dynamics ofn a t u r a l s e l e c t i o n .

In st ea d of imagining a gene contemplat ing var iou ss t r a t e g i es , an en t i r e ly equ iva len t way of cons ide r ing th e sameevolut ionary problem i s t o im ag ine th a t t he two dec i s i on r u l e s* gene 2 c o d e s f o r d e c i s i o n r u l e 2 which, on average, maximizesan i n d i v i d u a l ' s " i n c l us i v e f i t n e s s " -- h i s own re produ ct ives u c c e s s p l u s h i s e f f e c t s on t h e r e p r o d u c t i ve s u cc e s s of h i sr e l a t i v e s , e ac h e f f e c t d is c o un t e d by t h e a p p r o p r i a t e I, t h eco ef f i ci en t of r el at ed ne ss (Hamilton, 1964; Dawkins, 1982).


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a r e embodied i n di f f er e nt organisms. One then imagines atournament p i t t i n g Gene 1 (which codes for Decis ion Rule 1 )ag ai ns t Gene 2 (which codes fo r Deci sion Rule 2) .

In th e tournament , two ind i v id ua l s fac e the sameenvironment. That i s , i n t h e f i r s t g en er at i on b ot h i n d i v i d ua l shave t h e same number of, r e l a t i v e s , t h e same number ofoppo r tun i t i e s /pe r r e l a t i ve fo r con e r r i ng b e ne f i t s , and th e sames e t of p a y o ff s a s s o c i a t e d w it h p a r t i c u l a r o p p o r t u n i t i e s .I n d i v i d u a l 1 has gene 1, a nd the re fo re u s e s de c i s ion ru le 1;Ind iv i dua l 2 has gene 2 and t h e r e f o r e u s es d e c i s i o n r u l e 2.Before the tournar~~entt a r t s , g e n e s 1 and 2 e x i s t i n eq u a lnumbers i n t he popula t ion .

Using t h i s tournament, one can ask: After one generation,how many r e p l i c a s of gene 1 versus gene 2 e x i s t i n th epopu la t ion ? How many r ep li ca s of each ex i s t af t er n gene rat ion s?I f one were t o run a computer model of t h i s tournament, one wouldf i nd t ha t a f t e r one ge ne ra t ion the r e would be more re p l i c a s o fgene 2 than gene 1; t he magnitude of th e di ff er en ce between themi s gene 2 I s "s el ec t i ve advantage" over gene 1. T h i s magnitudew i l l depend on what payoff and opportunity parameters weresp ec i f ie d i n th e program used. Af te r n genera t ions , where n i s afun c t i on of th e magni tude of gene 2 ' s s e l ec t i ve advan tage i n th etourn ament' s "environmentn, one would f i n d t h a t gene 2 had "gonet o f i x a t i o n " : t h a t gene 1 would repr esen t a vanish ingl y smallf r a c t i on of the gene poo l , r e ga rd l e s s of the a bs o lu te s i z e of t hepopula t ion .

Using the same thought experiment one can ask otherquestions: Once gene 2 has become fi xe d i n a popula t ion, i s i t


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vu l ne r ab l e t o i nva s i on by a mut an t gene cod i ng f o r a differentd e c i s i o n r u l e ( i . e ., i s i t a n E v o lu t i o n a ri l y S t a b l e S t r a t e g y , a nESS)? I f gene 1 i s f i x e d i n t h e p o pu la t i on , i s it v u l n e r ab l e t oin va s io n by a mutant gene 21 w i l l gene 2 sweep the popula t ion ,or w i l l a s t a b l e polymorphism r e s u l t be tween genes 1 and 23 I s ag en e b e t t e r o f f i f i t cpdes f o r a mixed s t r a t eg y , one t h a t u s esdec i s i o n r u l e 2 under c e r t a i n c i r cums t ances , and some o t he rde c i s io n ru l e under o th er c i r cums tances? And so on.

I n o t h e r wo rd s, n a t u r a l s e l e c t i o n t h e o r y h as a gametheoretic s t r u c t u r e (Maynard Smit h , 1982) . Th i s f a c t can beu s e f u l l y a p p l i e d t o a n a n a l y s i s of s o c i a l e x ch an ge b etwee nu n r e l a t e d i n d i v i d u a l s .

I n t he example above, t h e dec i s i o n r u l e s gove rned au n i l a t e r a l a c t ; sh ou ld I , or should I no t , be ne f i t my r e l a t i v e byd o i n g a c t Z? I n c o n t r a s t , s o c i a l e xc ha ng e i n v o l v e s two a c t s :what I d o f o r you ( a c t 1 ) and what you do fo r me ( a c t 2 ) . Myd o i n g a c t 1 f o r you be ne f i t s you ( B( you) ) a t some cos t t o mys el f( C (me)) . Your doin g a c t 2 fo r me b e n e f i t s me (B(me) a t somec o s t t o y o u r s e l f ( C ( y o u ) ) . F ur th er mo re , t h e b e n e f i t t o you ofre ce iv i ng my a c t 1 i s g r e a t e r t h a n t h e c o s t t o you o f d o i n g a c t 2f o r me (B (you ) > C ( y o u ) ) ; l i k e w i s e , t h e b e n e f i t t o me ofr e c e i v i n g a c t 2 from you i s g r e a t e r t h an t h e c o s t t o m e of doinga c t 2 fo r you (B(me) > C(me)). A l l c o s t s a nd b e n e f i t s a r em ea su re d i n i n c l u s i v e f i t n e s s t er ms : C ( X ) and B ( X ) r e f e r t od e c re a s es and i n c r e a s e s i n t h e i n c l u s i v e f i t n e s s of i n d i v id u a l x( s e e f o o t n o t e , pa ge 1 3 2 ) . I f a c t s 1 and 2 ha ve t h i s c o s t / b e n e f i ts t r u c t u r e , we bo t h ge t a ne t be ne f i t by exchang ing a c t s 1 and 2.L e t ' s c a l l an i n t e r a c t i o n t h a t i s m u t u a l l y b e n e f i c i a l ,


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A t f i r s t b l us h , one m ig ht t h i n k t h a t n a t u r a l s e l e c t i o n wouldfav or th e emergence of psyc holo gica l mechanisms with de ci s i onr u l e s t h a t l e a d o rg an ism s t o p a r t i c i p a t e i n a s o c i a l e xchangewhene ver t h e a bo ve c o n d i t i o n s h o ld . A f t e r a l l , p a r t i c i p a t i o nwould r e s u l t , by d e f i ni $ i o n , i n a n e t i n c r e a s e i n t h e r e p l i c a t i o nof g e ne s u nd e r ly in g a te nd en cy t o p a r t i c i p a t e , a s compated t og e n e s u n d e r ly in g a t e nd en cy t o n o t p a r t i c i p a t e .

But the re i s a hi tc h: You can be ne f i t even more by chea t ingme. I f I d o a c t 1 fo r you , bu t you do no t do a c t 2 fo r me, the nyou b en ef i t more than i f we both cooperate . T hi s s i n g l e f a c tc r ea te s an enormous s tumbl ing b lock fo r t he evo lu t i on of so c ia lexchange, a problem th a t i s s t r u c t u r a l l y i d e n t i c a l t o one of t h emos t famous s i t ua t i o ns i n game theory : th e one move Pr i so ne r ' sDilemma (e .g. , T ri v er s , 1971; Axelrod & Hamilton, 1981; Axelrod,

The Prisoner 's Dilemma i s a game i n which mutual c oop era tio nwould b e n e f i t b oth p l a y e r s , b u t i t i s i n t h e i n t e r e s t o f eachp l a y e r , i n d i v i d u a l l y , t o d e f e c t , c h e at , o r s n i t c h on t h e o t he r .I t i s f r e q u e n t l y c o n c ep t u a li z e d a s a s i t u a t i o n i n which twopeople who have co l l abo ra t ed i n committ ing a c r ime a r e p reven tedf ro m com municating, w h i l e a d i s t r i c t a t t o r n e y o f f e r s e ac hi n d i v i d u a l a l i g h t e r s e n t en c e i f he w i l l s n i t c h on h i s pa r t n e r .However, th e payo ffs can repr ese nt a nyth ing fo r which both* Other models o f so c ia l exchange a r e pos s ib le , bu t they w i l l n o tc ha ng e t h e b a s i c c o n clu s io n of s e c t i o n 5.1: t h a t r e c ip r o ca t i on i snecessa ry fo r th e evo l u t io n o f so c i a l exchange. For example, t heP r i s o n e r s Dilemma assumes t h a t eno r c e a b l e t h r e a t s a n de n f o r c e a b l e c o n t r a c t s a r e i m p o s s i b i l i t i e s ( Ax el ro d, 1 9 84 ),a ss um pt io ns t h a t a r e f r e q u e n t l y v i o l a t e d i n n a t ur e . Thei n t r o d u c t i o n of t h e s e f a c t o r s would n o t o b v i a t e r e c i p r o c a t i o n --i n f a c t , t h e y would en o r c e i t .


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p la y e r s ha ve a s im i l a r p r e f e r e n c e r a n kin g: money, p r e s t i g e ,po in t s i n a game, even reprodu c t iv e success . A p o s s ib l e p a y o f fm a t r i x and t h e r e l a t i o n s h i p t h a t must e x i s t between v a r i a b l e s i sshown i n Fi gure 5 .1 .

Figure 5 .1 Payoff Schedule , Pr i son er s Dilemma...................................................................

.............................: me: R = +3 : me: S = -2 :

C :: you: R = +3 : you: T = +5 :me :..................eooo~.....: me: T = +5 : me: P = 0 :

D :: y o u : S = - 2 : you: P = O :

C = CooperateD = DefectR = Reward fo rmutual cooper a t i o nT = T em p ta ti on t o d e f e c tS = Sucker ' s payoffP = punishment form u ta l d e f e c t i o n

* For an i t e r a t e d game, R > (T+S)/2 . This i s t o p r ev e nt p l a y er I sf ro m " co o p e ra t i ng " t o maximize t h e i r u t i l i t y by a l t e r n a t e l yde fe c t in g on one ano ther .

Looking a t t h i s payof f ma t r ix , one might ask : "What 's th edilemma? I w i l l b e b e t t e r o f f , a n d s o w i l l you, i f we bot hc o o p e r a t e -- you w i l l s u r e l y r e c og n i z e t h i s and c o o p e ra t e w i thme. " However, if t h e r e i s onl y one move i n t h e game, i t i sa lw ay s i n t h e i n t e r e s t of e ac h p a r t y t o d e f e c t (Luce t R a i f f a ,1957) -- t h a t i s what creates the di lemma.

Le t 's say you and I a r e p lay ing a one move Pr i s on er ' sDilemma game. I would reason thus: "You w i l l e i t h e r c o o p e r a t e o rd e f e c t . I f you cooperate , then I ge t a h igher payoff byd e f e c t i n g , b e c a u s e T , t h e T em pt at io n t o d e f e c t , i s g r e a t e r t h a nR , the reward I would ge t fo r mutua l coopera t io n . I f you de fe c t ,t h e n I ge t a h igher payof f by a l s o def ec t ing , because P, th e


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Punishment f o r mutual de fe c t i on , i s g r e a t e r t h a n S , t h e S u c k e r 'spayoff I would g e t i f I coope ra te and you de fe c t . Ther efore , nomatter what you do, I am b e t t e r off de fe ct i ng ." Your rea son ingpr oc es s would be i d e n t i c a l , so we would both de fe ct , and we wouldboth ge t P, t h e Punishment fo r mutua l defec t ion . L e t ' s s a y t h epayo f f m a t r i x i n F i gu re 5'.1 r ep re sen t ed do l l a r s : i f youc o o p e r a t e , I g e t $5 f o r d e f e c t i n g i n s t e a d of $ 3 f o r c o o p e r a t i n g .I f you de fec t , I l o s e n o t h i ng by d e f e c t i n g i n s t e a d of l o s i n g $2by coopera t ing .

F igure 5 .2 shows t h a t t h e c o s t /b e n e f it s t r u c t u r e of a s o c i a le xc ha ng e h a s t h e same s t r u c t u r e a s a P r i s o n e r s Dilemma. If Icoope ra te on our agreement, you g e t B(you) fo r de fe c t ing , whichi s g r e a t e r t h a n t h e B( yo u) - C(you) you would g e t f o rcoope ra t i ng . I f I de fe c t on our agreement , you g e t no th ing f o rd e f e c t i n g ( t h i s i s e q ui va le nt t o o ur n o t i n t e r a c t i n g a t a l l ) ,which i s be t t e r t han th e C(you) lo s s you would i ncu r byc o o pe r at i ng . The p a y of f s a r e i n i n c l u s i v e f i t n e s s u n i t s -- t h enumbers l i s t e d a r e i n c l u d ed s im pl y t o r e i n f o r c e t h e a na lo g y wi thF i gu r e 5.1. I n a c t u a l i t y , t h e r e i s no reason why C(me) musteq ua l C(you) (o r B(me) = B(you) ) ; an exchange w i l l have thes t r u c t u r e of P r i s on e r ' s D ilemma a s l ong a s m u tua l coope ra t i onwould produce a ne t ben ef i t f o r bo th of us .

How can a s ystem of mutual co op er at i on emerge i n such as i t u a t i o n ? G iv en a n o p p o r t un i t y f o r e xc ha ng e, i f my d e c i s i o nr u l e was "Cooperate whenever B (me) > C (me)" and your r u l e was"Cheat" , th e genes unde rly ing my de ci s i on r u l e would soon bes e l e c t e d o u t . For ev e r y i n t e r a c t i o n w i th a " ch e a t e r " I wouldl o s e 2 i n c l u s i v e f i t n e s s p o i n t s , and t h e c h e a t e r would g a i n 5.

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By de f i n i t i on , t hen , my tendency t o cooperate would be se l e c t edout , and th e "Cheat" deci s io n r u l e would spre ad through thepopu la t ion ; th e number of gene ra t ions t h i s t ake s i s a func t ion o f

Figure 5.2 So ci al exchange se t s up a Pr is on er 's Dilemma

.........................................................:: me: B (me) - C (me) = +3 : me: C (me) = - 2

-me :.........................*.:*.ss...*.*....se~.......eso: m e : B(me) = +5 : me: O(me) = 0D :: you: C (you) = - 2 : you: O(y0u) = 0

B ( X ) = B e n e f i t t o XC ( X ) = Cost t o XO ( X ) = X ' s i n c l u s iv e f i t n e s s i s unchanged

how many c he a t e r s ve r sus ind i sc r i m ina t e coope ra to r s a r e i n thei n i t i a l p o p ul a t io n (Appendix B shows j u s t how qu ic kl y, give nsome rather generous assumptions) . I n pra c t i ce , a popu la t ion of"chea te r s" i s a popu la t ion of ind iv i dua l s who never p a r t i c i pa tei n s o c i a l exchanges; i f you chea t by not doing ac t 2 fo r me, andI chea t by not doing a c t 1 fo r you , then i n f a c t , we have no ti n t e r ac te d a t a l l -- w e have had no e f f e c t on one anothe r .

You m ig ht o b j e c t t h a t r e a l l i f e i s n ot l i k e a P r i s o n e r 'sDilemma, because r e a l l i f e exchanges ar e s imul taneous , face- to-fac e in t e ra c t io ns . You can d i r e c t ly see w he ther I am ab ou t t oche at you or not . I f I show up without the item I promised, thenyou simply do no t g iv e m e what I want. This i s c e r t a i nl y t r u e i na 2 0t h c en tu ry ma rke t economy, where money i s use d a s a medium of


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exchange. But i n na tur e , most exchanges a r e not , and cannot be ,s imul taneous . For example:

A common "item" of exchange between primates i s p r o t e c t i o nfrom conspecif i c s and pre dat ors . Two or more in di vi du al sd e ve l op c o a l i i o n a l r e l a t i o n s h i p s f o r mu tu al d e f en s e,aggress ion , o r p r o t ec t i on (e .g. , baboons : H a l l & DeVore, 1965;chimps: Wrangham, i n pr es s; de Waal, 198 2). I f you a r ea t t a c k e d , a n d I come t o your de fen se , th e r e i s no thi ng you cando , a t t h a t t ime , t a r r ep ay me. My repayment w i l l come when Iam at t a c k e d and you cdme t o my de fe ns e ( I hope!).

2. W e a r e f o r a g i n g f o r p at ch y resources. You f i nd a t r e e ladenwi th more f r u i t than you can ea t by you rse l f ; you g ive a shou tt o guid e me t o i t . There i s n o th in g I can do t o repay you ont h e sp ot . Your repayment w i l l come i n t he f u t u r e when I l e tyou know about a similar f ind -- you hope (e.g., bi r ds : Ward &Zahavi, 1973; bats: McCracken & Bradbury, 1981; chimps:Gooda l l , 1968 , 1971) .3 . I n c o o p er a t i v e h u n ti n g , t h e r e i s only one k i l l a t a ti m e, andusu a l l y on ly one o r two in d i v i du a l s ac tu a l ly make th e k i l l .Those who actually make the k i l l c la im the mos t, bu t theys h a r e t h e r e s t o f t h e m ea t w i th t h e o t h e r s on t h e h u n t ,t r u s t i n g t h a t th ey w i l l sha re one of t h e i r k i l l s a t somef u t u r e t ime. Again , repayment on th e sp ot i s im p o s s ib l e .4 . There i s mounting evidence that a baboon male forms "specia lr e l a t i o n s h i p s " w i t h a few l a c t a t i n g ( an d t h e r e f o r e i n e r t i l e )f e m a le s a nd t h e i r i n f a n t s : h e p r o t e c t s t hem fr om c o n s p e c i f i c sa nd p r e d a t o r s i n e xc ha ng e f o r s e x u a l a c c e s s when t h e f e m a le swean t h e i r i n f a n t s and become f e r t i l e a g a in ( e .g . , Smuts,1982; Strum, 1985). H i s repayment, by nec es s i ty , comes a t amuch la t e r t ime.The op po rt un it y fo r on-the-spot repayment i s r a r e i n na t ur e f o rs e v e r a l r e as o ns :1. The " i t em s " of e xc ha ng e a r e f r e q u e n t ly a c t s t h a t , o nc e d on e,cannot be undone (e .g . , p ro tec t io n f rom a t ta ck , a l e r t i n go t h e r s t o t h e p r e s e n c e of a f oo d so u r c e ) ;2. O p p o r tu n i t i e s f o r s im u l t an e o u s m u tu al a i d a r e r a r e b e c au s e t h en ee ds and a b i l i t i e s of o r g an is ms a r e c o n t i n u a l l y s h i f t i n g : t h efemale baboon i s n o t f e r t i l e when h er i n f a n t n ee ds p r o t e c t i o n ,

y e t t h i s i s when t h e ma l e 's a b i l i t y t o p r o t e c t i s of mostv a lu e t o h e r ;3 . F requen t l y , s imul taneous needs o r wi nd fa l l s cannot be tu rnedi n t o o p p o r t u n i t i e s f o r m ut ua l a i d : i f two i n d i v i d u a l s a r ea t t a c k e d s i m u lt a n eo u s l y, n e i t h e r i s f r e e t o h e lp t h e o t he r ; i ft h e y f i n d t wo fo o d s o u r c e s s im u l t an e o u sl y , n e i t h e r b e n e f i t sf ro m t h e o t h e r ' s w i n d f a l l .Thus, i n t h e absen ce of a widely acc ep ted medium of exchange,*


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most exchanges do c o n s t i t u t e a P ri s on er ' s Dilemma. You mustdeci de whether t o be ne f i t me or not wi thout any guarantee t h a t Iw i l l r e t u r n t h e f a vo r i n t h e f u t u r e . T h is i s why Trivers (1971)d e s c r i b e s s o c i a l e xchange i n n a t u r e a s " r e c i p r o c al a l t ru i s m. " Ibehave " a l t r u i s t i c a l l y " ( i . e. , I i n c ur a c o s t i n o r de r t o b e n e f i ty ou ) a t one p o i n t i n time, and you rec i p roc a te my a l t r u i s i c a c ti n t h e f u t u r e . I f you d o, i n f a c t , r e c i p r o c a t e , t h en our" r ec i p ro c a ll y a l t r u s i t i c " i n t e r a c t i o n i s p r o p er l y d e s c r i b e d a s a nins t an ce of de layed m utua l bene f i t : ne i the r o f us has incur r ed ane t c os t , bo th of us have ga ined a ne t ben e f i t . O bv ious ly ,however, i f only one in te ra c t io n i s i nvo lved -- t h a t i s , i f wea r e pla yi ng a P r i s o n e r ' s Dilemma game with only one move -- Iwould be a f o o l t o r e c i p r o c a t e y ou r a l t r u i s t i c a c t , and you,knowing t h i s , would be a f oo l t o do i t i n t h e f i r s t p la ce . So wea r e back t o s q u ar e 1: mutual defec t ion i s i n both of ouri n t e r e s t s .

Se l ec t ion p ressu r es change r ad ic a l ly when ind iv i dua l s p l ay as e r i e s of Pr i so n er 's Dilemma games. Mutual coop erat ion -- andt h e r e f o r e s o c i a l e x c h a n g e -- can emerge between two p l a y e r s when1) t h e r e i s a hi gh p r o b a b i l i t y t h a t t h e y w i l l meet again, 2 )n ei th er knows fo r s u r e ex ac tl y how many tim es the y w i l l meet,**

* Indeed, such f ac to r s a r e exac t ly why i t i s so use fu l t o have amedium of exchange. I d o n ' t h av e t o b e a b l e t o p r ov i de t h ep a r t i c u l a r goods or se rv ic es you want because you can conv ertmoney from me i n t o an yt hi ng . Fur ther more , money pe rm it s asimultaneo us exchange, i n which I can, i n f a c t , wit hol d my moneyi f I se e th a t you in t e nd t o chea t me, and v i ce ve r sa .** The game "u nr av el s" i f the y do. I f we bot h know we a r e pl ay in gt h r e e games, then w e both know w e w i l l mutual ly de fe c t on th el a s t game. I n pr ac t i ce , then, our second game i s our l a s t game.B u t we know t h a t we w i l l , t he r e fo re , m utua lly de f ec t on th a tgame, so , i n pr a ct ic e, we a r e pl ayi ng only one game. The argumenti s g e n e r a l t o any known, f i x e d number of games (Luce & Raif f a , 1957) .


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and 3) t hey do no t va lu e l a t e r payoffs by t oo much l e s s thane a r l i e r p a y o f f s ( Ax el ro d & Hamilton, 1981; Axelrod, 1984). I fyou and I a r e making a se r i e s of moves ra t he r than ju s t one, yourbeh av io r on one move can i nf l ue nc e my behavi or on f u t u r e moves.I f you d ef ec t when I coopera ted , I c an r e t a l i a t e by d e f e c t i n g onthe next move; * i f you s oop era te when I have, I can reward you byc o n t in u i n g t o c oo pe ra t e. I n a n i t e r a t e d P r i s o ne r I s Dilemma, asystem can emerge t ha t has inc en t ive s fo r coopera t ion andd i s i n c e n t i v e s f o r d e fe c ti o n.

For example, cooperat ion can be se le ct ed fo r i f it i sgoverned by a dec is i on r u l e th a t says : "Coopera te wi thin di vi du al s who have cooperate d w i t h me i n th e pa s t ; de fe c t w i t hin d i v id ua l s who have a h i s t o r y of de fec t i on ." Using th e payoffm a tr i x i n F i gu re 5 . 2 , i t i s c l e a r t h a t a s t r a t e g y l i k e t h i s coul dbe se le ct ed over an "a lways chea t" s t ra te gy . The mutualc o o p e r a t o r s would g e t s t r i n g s of +3 i n c l u s i v e f i t n e s s p o i n t s ,p ep pe re d w i th a few - 2s fr om a f i r s t t r i a l wi th a c h e at e r ( a f t e rwhich th e coopera tor ceases t o coopera te w i t h t h a t i n d i v i d u a l ) .In c on t ra s t , mutua l de fe c t o r s would ge t s t r i ng s of z e ro s ,

* I n n a t ur e , I c an a l s o r e t a l i a t e by i n f l i c t i n g a c o s t on youthr ough t he use of vi ol enc e. However, i f I c an , r e l i a b l y , d ot h i s , - t h e g a m e i s no longer a Pr is on er 's Dilemma. vi ol en tr e t a l i a t i o n i s a " t a x" on de fe c t ion th a t w ipes ou t the i nc e n t iv et o d ef e c t ( i . e. , T minus R ) . I f T J( R, t he n t h e s i t u a t i o n nolonger presents a dilemma -- we both have an in ce nt iv e t ocoop erat e and no in ce nt iv e t o cheat . The key word i n th e aboves c e n a r i o i s re l i a b lv . From a "ve i l of igno ranc e" a s t o there la t i ve s t r e ng th of two ind iv id ua l s , on a ve rage , ha l f the t i m e I( t h e c h e a t e d o n) w i l l be a b l e t o i n f l i c t a c o s t on you, and h a l fth e t ime you ( t he c he a te r ) w i l l be a b l e t o i n f l i c t a c o s t on me.There fore , it i s by no means cl e ar t h a t th e use of vi ol enc e i st h e most c o s t e f f i c i e n t way t o f o s t e r c o o pe r at i o n, e s p e c i a l l y i na one move game. Of cour se, most ani mal s ar e not a c t i n g from av e i l of ignorance, and one would expect them t o as se ss th e i rr e l a t i v e s t r e n g t h a nd a d j u s t t h e i r s t r a t e g i e s a cc or di ng ly .


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p ep pe re d w i t h a f ew +5 s f ro m a n o c c a s i o n a l f i r s t t r i a l w i t h acoopera to r ( a f t e r which the coopera to r never c oopera te s w i th t ha ti n d i v i d u a l a g a i n ) .

A number of s t r a t e g i e s p e r m i t t i n g s e l e c t i v e c o o p er a t io n a r ep o s s i b l e , b u t o ne t h a t h a s been p a r t i c u l a r l y s u cc e ss f u l i n r e c e n ti n v e s t i g a t i o n s i s c a l l e d T IT FOR TAT (Axelrod & Hamilton, 1981;Axelrod, 1984) . I t i s a ve ry s im ple s t r a t e gy i n which : 1) Icoop era t e on the f i r s t move, and 2) I do whatever you d id on th epr ev io us move. I f you co op er at e on move 1, then I coopera te onmove 2; i f you d e f e c t on move 1 , then I defect on move 2. T I TFOR TAT c an be u se d t o i l l u s t r a t e t h e s e l e c t i v e a dv an ta ge ofs e l e c t i v e c o op e ra t io n .

Table 5.1 i s designed t o gi ve you an i de a of how a TI T FORTAT d e c i s i o n r u l e s t a c k s up a g a i n s t a n " al wa ys c h e a t " d e c i s i o nr u l e (CHEAT) and a m ixed s t r a t egy ru le (MIXED) i n a round ro bintournament. The mixed st ra te gy r u l e i s a T I T FOR TAT programt h a t s l i p s i n some c h e a t i n g on t h e s i d e . A f t er a m u tu a ll ycooperat ive move, i t t r i e s t o r ac k up p o i n t s by d e f e c t i n g. I f i ts uc ce ed s i n e a r n i n g T, it immedia tely " apologizes " fo r i t sdef ec t ion by coope ra t in g on the next move, i n an a t tempt t ore s t o r e m utua l coopera t ion . I f MIXED does no t succeed i n ea rn ingT (i. . , i f i t s p a r t n e r a l s o d e f e c t e d ) , it " r e t a l i a t e s " b yde fe ct in g on the next move.

A s you can see, TIT FOR TAT e a r n s more p o i n t s i n t h i s roundr o b i n th a n e i t h e r MIXED o r CHEAT. Because po in ts s tan d fo rr e p l i c a s of g en es c od in g f o r e ac h d e c i s i o n r u l e , t h i s means t h a tT I T FOR TAT genes would spre ad through th e popula t ion , eve ntu al l yd i s p l a c i n g MIXED and CHEAT. T h is r e s u l t i s n ot a n a r t i f a c t of

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T a b le 5.1 Round ro b i n tou rna me nt p i t t i n g TIT FOR TAT (TFT) v e rs u s CHEATv e r s u s MIXED

TFT v. MIXED SUBTOTALS MIXED v. MIXED

TFT v. CHEAT CHEAT v. CHEAT MIXED v. CHEAT

CONTENDER 'S SCORES: opponen t sM IXED TFT CHEAT.....................*.....*. * * . * * * * . . . * * * * . . . .:TIT FOR TAT: 21 + 30 + - 2 = 49 :c o n t e n d e r s :CHEAT: 5 + 5 + 0 = 1 0 :

:MIXED: 3 + 21 + - 2 = 22 ::.........................*~.*.*.*~*~~~...***.t h e p a r t i c u l a r s t r a t e g i e s i t was p i t t e d a g a i n s t i n T ab le 5 .l.Rober t Axe l rod conduc ted a round rob in computer tourname nt


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i n wh ic h TI T FOR TAT was p i t t e d a g a i n s t 62 o t h e r e n t r i e s . A l le n t r i e s w er e s u b m i tt e d by s o p h i s t i c a t e d s t u d e n t s of t h ePr i s on e r ' s Dilemma, i nc l ud i n g p r o f es so r s of p sycho l ogy , b i o l ogy ,and p o l i t i c a l scie nc e. TIT FOR TAT ach ieve d t h e hi gh es t avera gescore (Axel rod , 1984) . I t s s u c c e s s ap p e a r s t o b e du e t o f o u rf a c t o r s :1. TIT FOR TAT i s "n i ce" : i t never d e f e c t s f i r s t2. When i t s opponent defec t s , T IT FOR TAT r e t a l i a t e s ; h enc e T I TFOR TAT i s n o t e x p l o i t a b l e3. T IT FOR TAT i s " f o r g i v i n g " : i f i t s o p p o n e n t i n i t i a t e sc o o p e r a t i o n a f t e r h a v i ng d e f e c t e d , TIT FOR TAT c o o p e ra te s onthe next move; it d oe s n o t g e t c au g ht i n e n d l e s s c h a i n s o f

r e c ri m i n at i o ns ( a s i n MIXED v. MIXED)4 . TIT FOR TAT i s s o c l e a r an d c o n s i s t e n t t h a t , o n ce e n c ou n te r ed ,

it i s e a s i l y r e co g n iz e d , a nd i t s n o n - e x p l o i t a b i l i t y i s e a s i l ya p p r e c i a t e d .The a u t h o r s o f a l l e n t r i e s s u b m i t t e d knew t h a t TIT FOR TAT hadwon a prev ious tournament of 1 2 e n t r ie s . Fur thermore, th ey hadbeen g i ve n a n e x t e n s i v e a n a l y s i s of t h e p r o p e r t i e s t h a t h ad l e dt o i t s succes s . Some au t h o r s submi t t ed mixed s t r a t e g i e s t h a tusu a l l y p l ayed TIT FOR TAT, but t r i e d t o ge t away wi th occ as i ona lc h e a t i n g . O t h e rs , g u e s s in g t h a t t h e a n a l y s i s p r ov i de d wouldprompt many au t h o r s t o submit "n i ce" s t r a t e g i e s , submi t t ed" e x p lo i t at i ve " s t r a t e g i e s -- s t r a t e g i e s d es ig ne d t o t a k ea d va nt ag e of " n i c e n e n t r i e s . I n g e n e r a l , t h e e x p l o i t a t i v es t r a t e g i e s won an o c c as i o n a l b a t t l e b u t l o s t t h e wa r, e a r n i n g t h el o w e s t a v e ra g e s c o r e s i n t h e rou nd r o b i n .

Ax el r od a l s o c o nd u ct ed a s i m u l a t i o n of n a t u r a l s e l e c t i o nover t ime : The more po i n t s a s t r a t eg y ea r ned i n one "gen e r a t i onn( ro u nd r o b i n ) , t h e more " c o p i es " of t h a t s t r a t e g y c omp ete d i n t h enex t "gene r a t i on . " Over t h e gen e r a t i on s , TI T FOR TAT an d o t h e r


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n ic e -b u t - re t a l i a to ry s t r a t e g i e s came t o d omin ate th e p o pu la t ion .The e x p l o i t a t i v e and mixed s t r a t e g i e s e v e n tu a l ly went "e x t in c t . "TIT FOR TAT alw ays had t h e l a r g e s t s ha r e of t h e "gene poo ln : byth e 1 0 00 th and l a s t g e n e ra t io n , i t s r e p r e s e n t a t i o n i n t h e g enepool was s t i l l gr owi ng a t a f a s t e r r a t e t h a n t h a t of any o t h e rs t r a t e g y .

Oth er c a l c u l a t io n s d e mo ns t ra te d t i f a t a v e ry sma l l c l u s te r ofT IT FOR TATTER6 can invad e a pop ul at io n of ch ea te r s, even i f ver yfew of t h e i r in t e r ac t i on s a r e wi th each o ther . Futhermore, i tcan be mathematical ly demonstra ted t h a t T I T FOR TAT i s anEv o lu t io n a r i l y S ta b le S t ra t e g y (ESS) : no "mutantn s tr a t eg y caninvade a population composed primarily of T I T FOR TATTERS, e i t h e rs i ng ly o r i n smal l c l us te rs . The average performance of a TITFOR TATTER i n a pop ula t ion of i t s f e l l o w s i s higher than theaver age performance of any po s s i bl e newcomer.

The d e t a i l s of TIT FOR TAT a r e n o t what i s important aboutt h i s s t o r y . The key po i n t , which TIT FOR TAT i l l u s t r a t e s , i st h a t a coope ra t ive s t r a t eg y can invade a popula t ion of non-c o o p e r a t o r s i f , and o nl y i f , i t coopera tes wi th o the r coope ra to rsand chea ts on chea te rs . Indiscrlmlnate. --&selected fpr Fn w. e humans ha ve t h e a b i l i t y t ocoopera te f o r mutual benef i t . Th is capa c i t y cou ld no t haveevolved un less it i n cl u de d a l g o r it h m s f o r d e t e c t i n g -- and beingprovoked by -- chea t ing .


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5.2 So c i a l exchange and th e P le i s toce ne env ironment :The e c o lo g i c a l c o n d i t i o n s n e c e s sa r y f o r t h e e v o lu t i o n ofso c ia l exchange were mani fes t dur i ng hominid evo lu t ion ;homin id behav iora l eco logy fu r th e r co ns t r a i ns acomputa t iona l theory of so c i a l exchange.

Coopera tion can ev olv e only when 1 ) th e r e a r e manys i t u a t i o n s i n which i n d i y i d u a l s c an b e n e f i t e ac h o th e r a t lowc o s t t o th e m s elv e s ( i . e . , a n i t e r a t e d P r i s o n e r ' s Dilemma game i sp o s s i b l e ) , a n d 2) t h e p r o b a b i l i t y of two i n d iv i d u a l s m e et i nga g a i n i s s u f f i c i e n t l y h i gh . * The p r o b a b i l i t y t h a t twoi n d i v i d u a l s w i l l meet again i s i n c re a s ed i f t h e i n d i v i d u a l s a r elo n g - l i v e d a nd h av e low d i s p e r s a l r a t e s . T he se l i f e - h i s t o r yf a c t o r s a l s o i n c re a s e t h e number of s i t u a t i o n s f o r mu tu al h e l pt h a t t wo i n d i v i d u a l s a r e l i k e l y t o e n co u nt e r. The e c o l o g i c a l andl i f e - h i s t o r y f a c t o r s c h a r a c t e r i s t i c of t h e human e nv ir on me nt ofe v o lu t i on a r y a d a pt i v en e s s f u l f i l l t h e c o n di t i on s n e ce s sa r y f o rt h e e v o lu t i o n of c o o p er a t i o n . P l e i s t o c e n e h u n t e r - g a th e r er s weren o t o nl y lo n g - li v e d, b u t t h e y l i v e d i n s m a l l , r e l a t i v e l y s t a b l ebands . Thus, the p ro ba b i l i ty was h igh t h a t an ind iv i dua l you hadhe lp ed would be around when you needed hel p. Moveover, i n a l lp r o b a b i l i t y t h e s e i n d i v i d u a l s , l i k e modern h un t e r- g a th e r er s , werec l o s e l y r e l a t e d ; k i n s e l e c t i o n c an be a t remendous a i d t o t h eevo l u t io n of coopera t io n (Tr ive rs , 1971 ; Axel rod & Hamilton, 1981).

The i n t e l l e c tu a l c a p a c i t i e s of e a r l y h om in id s a l l o we d themt o g e n e r a t e many s i t u a t i o n s f o r w hich c o o p er a t i o n p a id o f f . Themos t impor tan t of th es e was th e capa c i ty t o make and use t oo ls ,a nd t h e c a p a c i t y t o g e n e r a t e n o ve l b e h a v io r a l p r o c ed u r e s t o-* For example, TIT FOR TAT i s an ESS i f , and o nl y i f , t h ep r o b a b i l i t y t h a t two i n d i v i d u a l s w i l l meet again i s g r e a t e r t h a nt h e l a r g e r of th es e two numbers: (T-R)/(T-P) and (T-R)/(R-S)(Axelrod, 1984).


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ac hi ev e a goal. The ex pl oi ta ti on of a new savannah and woodlandn iche -- made po ss ib le by t oo l use -- a ll ow ed i n d i v i d u a l s t oacq ui r e food i tems too l a r ge t o be consumed by a s in gl eindividual (Tooby & DeVore, 1985) .* T h i s c r e a t e d t h e p e r f e c to p p o r t u ni t y t o p ro vi d e a l a r g e b e n e f i t t o an ot h er i n d i v i d u a l a t ave ry low co s t t o onese l f . There i s v i r t u a l l y no c o s t t o s h a r in gfo od t h a t you can no t consume anyway, and tomorrow you may be th eone who has found no food. Fo ss i l evidence in di ca te s t h a tP le i s tocene hun te r -ga the re r s , l i k e t he i r modern cou nte rpa r ts ,engaged i n ext ens ive food-sharing (e.g. , I s sa c , 1978).S i m i l a r l y , t h e c o s t of s h a r i n g t o o l s i s low compared t o t h ebenefi ts one can garner through using them -- and the cos t ofsh ar in g inf orm ati on abou t t o o l making may be even lower.

When combined wi th th e i r c apa ci t y t o opp or tu ni s t ic a l lymanipulate th e environment through to ol use , our an ce s t or s 'a b i l i t y t o g e n e r a t e n ov el b e h av i o ra l p r oc ed ur es ** c r e a t e ds i tu a t io ns i n which coord ina ted , coopera t ive behav io r cou ldproduce va st pay offs . Perhaps one of t he best examples a r e th e"p ro f i t s m t o be made through coopera t iv e hunt ing . Actingt o g e t h e r , s e v e r a l armed men ca n b ri ng down a wooly mammoth;ac t i n g alo ne, a s i ng l e armed man cannot .

These cond i t ions s e t t he s t a ge fo r t he coevolu t ion of at i g h t l y interwoven complex of ada pta t io ns t h a t made coop erat i on* And which could not be s to re d fo r l a t e r use wi thout s po i l in g --ea r l y hominids had no re f r ig er a t io n!** An a b i l i t y th a t some o the r p r im a tes a l so possess, t o a l e sse rex te nt . For example, de Waal (1 982 ), shows p i c t u r e s of chimpswho h av e di s c o v e r ed t h a t t h e y c an g e t p a s t a n e l e c t r i f i e d f e n c esurrounding a t r e e wi th ed ib le leave s . One chimp holds a la r geb ra nc h a g a i n s t t h e t r e e a s a l a d d e r , w h il e an o th e r c l i mb s i t i n t oth e t re e . The chimp i n th e t r e e then throws ju icy leave s down t oh i s compatr io ts on t h e ground.


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more and more p r o f i t a b l e (Tooby & DeVore, 198 5). Coo per ati vehun t ing p rov ided a compact and nu t r i t i ou s food source th a tp r ov i de d an e f f i c i e n t means f o r ma le s t o i n v e s t i n o f f s p r i n g ;l e a d i n g t o mechanisms t o i n s u r e t h e i r p a t e r n i t y ; l e a d i n g t o (1)more c l o s e l y r e l a t e d s u b s e t s of i n d i v i d u a l s w i t h i nbands , c r ea t i ng l a rge r pa yof f s fo r coo pera t ive behav io r s and moreg ro up s t a b i l i t y ( wh ic h c r e a t e s e ven more o p p o r t u n i t i e s f o rc o o p e r a t i o n ) , a n d ( 2 ) even g re a te r payof f s fo r ma le pa r en t a linves tm en t i n o f f sp r ing ; l ead ing t o more m ale pa re n ta linves tment ; which a l lows la rg er b ra in s and longer per iods f orm a t ur a t io n a n d l e a r n i n g ; l e a d i n g t o more e f f i c i e n t c o o p er a t io nand to o l use , and th e r e f o r e t o even more nu t r i t i o us food sourcesfrom both hunting and gathering; making i t more e f f i c i e n t t odevote metabol i c resour ces t o bra in over brawn.. . and so on, eachc o n d i t i o n c i r c l i n g b ack t o a mp l if y t h e e f f e c t s of t h e o n es b ef o r eit , u n t i l t o da y c o op e r at i o n f o r m ut ua l b e n e f i t i s a pe rvas ive andi n e x t r i c a b l e a s p e c t of a l l human c u l t u r e s .

R e co n st r uc t io n of t h e e x a c t c a u s a l c h a in t h a t l e d t o t h eevol ut io n of coopera t io n i s s t i l l a matt er of debat e (c f . Kinzey,198 5). The most important poin t i s t h a t t h e P l e i s t o c e n e h u n t e r -ga th er er environment i n which we evolved provi ded manyo p p o r t u n i t i e s f o r i n d i v i d u a l s t o b e n e f i t f rom m ut ua l c o o p e ra t i on .

The pe c u l i a r i t i e s of hominid behaviora l ecology plac e somes p e c i e s - s p e c i f i c c o n s t r a i n t s on a co m pu t at i on a l t h e o r y of s o c i a lexchange i n humans. Exchange i n most prim ate s i s r e s t r i c t e d t ore la t i v e l y few " i tems" : food, sex ual acce ss , d efense , grooming.The fewer t he i tems f o r exchange, th e more " i tem-specif i c n th ealg ori thm s re gu la t i ng exchange can (and shou ld) be: What coun ts


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a s " e r ro r " -- c h e a t i n g o r u n d e r - r e c ip ro c a t i n g -- can be morec l o s e l y d e f in e d , i n c r e a s in g t h e a c cu r ac y of o n e ' s m e n ta laccount ing sys tem and th e accuracy of re fe renc e ( se e se c t io n5 .3 ) . I n c o n t r a s t , human a l g o r i t h m s f o r r e g u l a t i n g s o c i a lexchange should be a b le t o hand le a wide and ever-chang ing a r ra yof "i temsn f o r exchangw. to o l s , inormation about tool-making,p a r t i c i p a t i o n i n o p p o r t u n i st i c a l l y - cr e a t e d , c o o rd i n at e db e h a v i o r a l r o u t i n e s . T h i s s u g g e s t s t h a t ou r a l g o r i t h m s f o rr e g u l a t i n g s o c i a l ex ch an ge -- and t h e a s s o c i a t e d c o g n i t i v ec a p a c i t i e s th e y r e q u i r e t o f u n c t i o n p r op e rl y -- w i l l have somehuman-spec i fic p r op er t i e s . These w i l l be d i s c u s s e d i n t h e n e xts e c t i o n .

5.3. A computa t iona l theory of so c i a l exchange

D avid Marr h a s a r g u ed t h a t t h e f i r s t a n d most im p o rt a n t s t e pi n und er s t a n d in g a n i n ormation-processing problem i s deve lop inga " the ory of t h e computa tion" (Marr, 1982; Marr & N is h ih a r a ,1978). T h i s t h e o r y d e f in e s t h e n a t u r e of t h e pr ob le m t o bes o l v ed ; i n s o do in g, i t a l l ow s one t o p r e d i c t p r o p e r t i e s t h a t anyalgo r i th m ca pab le of so lv in g th e problem must have.C o mp u ta t io n al t h e o r i e s i n c o r p o r a t e " v a l i d c o n s t r a i n t s on t h e wayth e w o r ld i s s t r u c t u r e d -- c o n s t r a i n t s t h a t p ro vi de s u f f i c i e n ti n f o r m a t io n t o a l l o w th e p r o c e s s in g t o s u c c ee d " (Marr &Nish ihar a , 1978 , p. 4 1 ) .

For h umans, an e vo lve d s p e c i e s , n a tu r a l s e l e c t i o n i n ap a r t i c u l a r e c o l o g i c a l s i t u a t i o n d e fi n e s and c o n s t i t u t e s " v a l i dc o n s t r a i n t s on t h e way t h e w o r ld i s s t r u c t ur e d n f o r a p a r t i c u l a ra d a p t i v e i nf o r m a t io n p r o c e s s in g p ro bl em . I n t h e c a s e of s o c i a l


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exchange, th e eco lo gi ca l and game-theoretic as pe ct s of hominidso c ia l exchange discussed above provide the ingre dien ts f or thecons t ruc t ion o f j u s t such a computat ional theory. Acomp utational theory of s o c ia l exchange mu s t be powerful enought o (1) permi t t he r ea l i za t i on o f a "poss ib le" soc ia l exchanges t r a t e g y , t h a t i s , a s t r a t e gy th a t can be se l ec t ed fo r , and ( 2 )exc lude " imposs ib l e" s t r a t eg i es , t ha t i s , s t r a t e g i e s t h a t can notbe se l e c t e d fo r .

The a b i l i ty to engage i n a p o s s i b l e s t r a t e g y of s o c i a lexchange presupposes t he a b i l i t y to sol ve a number ofin ormation -processing problems. The problems most sp e c i f i c t osocial exchange w i l l be inco rpo rat ed in t o a "grammar of s o c ia lcon t r ac t sn i n t he second ha l f of t h i s s ec t ion . A grammar ofs o c i a l c o n t a c t s i s th e s e t of assumptions about th e ru le sgovern ing a pa r t i cu l a r so c i a l exchange th a t mu s t somehow beincar nated i n the psychologica l mechanisms of both pa r t i c i pa nt s .I t i s th e aspe ct of th e computat ional theory of so c ia l exchangemost re le va nt f o r under st an di ng performance on th e Wasons e l e c t i o n t a s k .

However, th e grammar of s o c i a l c o n tr a ct s does not exh austth e s e t of information processin g problems posed by s o ci a lexchange. The a b i l i t y t o s u c c e s s f u ll y p a r t i c i p a t e i n s o c i a lexchange al so re qu ire s a number of ot he r , ass oc iate d cog ni t iv eca p ac it ie s, some of which a r e necessa ry i n a wide range of oth ere v o l u t i o n a r i l y c r u c i a l s o c i a l i n t e r a c t i o n s , l i k e m at in g, p a i r -bonding, paren t ing, and aggress ion. Before progress ing t o thegrammar of s o c ia l co nt ra ct s and i t s impl ica t ions for per formanceon the Wason se l ec t ion t a sk , f i v e a s soc i a t ed co gn i t i v e cap ac i t i e s


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e n t a i l e d by s o c i a l ex ch an ge w i l l be examined:1. The a b i l i t y t o r e c o gn i z e many d i f f e r e n t i n d i v i d u a l s2. The a b i l i t y t o remember a s p e c t s of o n e ' s h i s t o r y ofi n t e r a c t i o n w it h d i f f e r e n t i n d i v i d u al s3 . The a b i l i t y t o c ommu nicate o n e ' s v a lu e s t o o th e r s .4 . The a b i l i t y t o model t h e v a l u e s of o t h e r i n d i v i d u a l s .5 . The a b i l i t y t o view i t e m s one p e r c e iv e s a s c a u s a l l y c o nn e ct e dt o b i o l o g i c a l l y s i g n i f i c a n t v a r i a b l e s a s c o s t s and b e n e f i t s ;human a l g o r i t h m s r e g u l a t i n g s o ~ i a 1 ' ~ e x c h a n g ehould not be tooc l o s e l y t i e d t o p a r t i c u l a r i t e m s of exchange.

Undoubtedly, a cl ev er programmer could de sig n many d i f f e r e n ta lgo r i th ms capab l e of so l v in g the se p roblems. I t i s evenpo ss ib le t h a t one or two of them cou ld be so lved , a l b e i t s lowlyand c lums i ly , by domain gener a l mechanisms l i k e as so c i a t iv e ne t s .But t o demons t rate th a t such mechanisms cou ld , i n theory , so lv eth es e problems would be t o m i s s th e poin t . The poi nt of us in gn a t u r a l s e l e c t i o n t h e o ry i n c r e a t i n g co mp ut at io na l t h e o r i e s i st h a t i t a l lo ws you t o s pe c i fy a s e t of p roblems t h a t humans oughtt o be a b l e t o s o l ve q u i ck l y , r e l i a b l y , e f f i c i e n t l y , and w i th o ute x p l i c i t i n s t r u c t i o n . T he se a r e p ro ble ms f o r which n a t u r a ls e l e c t i o n s h o u ld h av e p ro du ce d s p e c i a l i z e d , domain s p e c i f i cDarwinian a lgo r i th ms: modules i n Fodor 's or Marr 's t erminolo gy,mental organs o r c og ni t i ve competence8 i n Chomsky's termin ology,a d a p t a t i o n s i n t h e t e rm ino log y of e v o lu t i o n a r y bio lo gy . I t i st h e p re su mp ti on t h a t n a t u r a l s e l e c t i o n h a s d e s ig n ed p s y ch o l og i c almec hanisms t h a t a r e p a r t i c u l a r l y good a t s o l v in g t h e s e p ro bl em st h a t c a r r i e s i m p l i ca t i o n s f o r t h e s tu d y of a t t e n t i o n ,communication, t he orga niz at i on of memory, im pl i c i t in fe re nc e,a n d l e a r n in g . I s h a l l b r i e f l y s k e tc h a few of t h e s ei m p l i c a t i on s , o c c as i o na l l y c i t i n g r e l e v a n t d a ta .

151


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c a l l e d prosopagnosia (Gardner , 1974). Carey & Diamond (1980)pr es en t and review an impressi ve arr ay of evidence from a wideva r i e t y of sources sugges t ing t h a t humans have inna te ly sp ec i f i e dface -encod ing schemas. We a r e a l so good a t i den t i fy ingi n d i v i d u a l human g a i t s ( C u tt i n g , P ro f f i t t , & Kozlowski , 1978;Kozlowski & C ut t ing , 1477) .

P r o p o s i t i o n 2 . One must be a b l e t o refnember some a s p e c t s of t h eh i s t o r i e s o f o n e 's i n t e r a c t i o n s wi th d i f f e r e n ti n d i v i d u a l s .F i r s t , one must be a b l e t o r e c o gn i z e t h a t a p r e v i o us

i n t e r a c t a n t i n a s o c i a l e xchange i s , i n f a c t , a p re vi ou sin te ra c t an t , and no t , fo r example, a s t r ang er , a m ate, o r anof f sp r ing . Second , once an ind iv idua l has been id en t i f i ed a s ap r ev i ou s i n t e r a c t a n t , i n ormation regardi ng whether t h a tin di vi du al has been a coop erat or or a ch eat er must becomeac ce ss ib le t o th e dec i s ion p rocedures . Th i rd , one needs an"accounting system" for keeping track of who owes who what. A sd i sc usse d i n sec t io n 5.1 , most P le i s tocene so c i a l exchangesi n v o l v e d " r e c i p r o c a l a l t r u i s m " -- exchanges i n whichrec ipr oca t io n was delayed, not s imul taneous . I n a s imul taneous ,face-to-fa ce exchange, i f you see t h a t the oth er person has comeprepared t o def ec t , you s imply wi thhold what t h a t person wants .*There i s no nee d t o remember how much you owe or a r e owed,because the re i s no owing: each tr an sa ct io n i s e i t h e r a completeexchange or a com plete de fe c t ion . The po te n t i a l fo r chea t ingi s much high er , however, i n exchanges i n which re ci pr oc at io n i s* One would expe ct pe ople t o assume, i n th e absence ofi n f or m a ti o n t o t h e c o n t r a r y , t h a t s uc h i n t e r c o n t i n g e n t b eh av io ro c c ur s i n f a ce - to - fa c e i n t e r a c t i o n s . They should be more likelyt o su spec t someone of in t end ing t o chea t i n de layed ben ef i tt r a n s a c t i o n s .


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delayed; once you have confer r ed a ben ef i t , you cannot tak e itback. To be a b l e t o " ca l l i n your markers" , you must be a bl e t okeep t r a c k of who owes what. Consequently, th e ca pa ci ty fo re ng ag in g i n t r a n s a c t i o n s i n which r e c i p r o c a t i o n i s delayedre qu ir es a mental a ccounting system fo r keeping tr ac k of who oweswho what (no te: Prop osi, t ion 5 a l s o a p p l i e s t o t h i s a cc ou nt in gsystem) .

The ext ent of t he h i s t or y of in te ra c t io n t h a t must becomea v a i l a b l e t o t h e d e c i s i o n p r oc ed ur e t h a t r e g u l a t e s wh et he r youa g r e e t o p a r t i c i p a t e i n a p a r t i c u l a r s o c i a l e xchange ( an d wh et he rany of thes e f a c t s need be consc ious ly r eca l l e d ) w i l l depend onth e de t a i l s of th e pa r t i cu la r d ec i s i on procedure humans haveevolved. TIT FOR TAT r e q u i r e s only t h a t t h e l a s t t r a n s a c ti o nw i th each in t e ra c t an t be r eca l l ed . But TIT FOR TAT o p er a te s i n ah igh ly cons t r a in ed and un iform un ive r se w here a l l t r an sac t io nsa r e s im ult aneous , t he same payof f m a t r ix app l i e s t o eacht r a n s a c t i o n , and t h e s i z e of t h e p a yo f f s f o r bo th p l a y e r s i se q u a l w i t h i n e ac h t r a n s a c t i o n . I n c o n t r a s t , p a yo ff m a t r ic e s i nt h e r e a l wo rl d a r e al wa ys i n f l u x , and p a r t of t h a t f l u x i scaused by the ne go t i a t ive s k i l l s o f t h e i n d i v i d u a l s i n vo l ve d .Moreover, violence i s p o s s i b l e i n t h e r e a l w or ld : e xc ha ng es i t u a t i o n s wi t h i n d i v i d u a l s who can r e l i a b l y u se v i o l e n c e t o g e tt h e i r way do n o t n e c es s a r i l y f i t t h e c o n s t r a i n t s of a P r i s o n e r ' sDilemma. Thus, an a lgor i thm be t t er adapted t o cond i t io ns i n th er e a l w orld m ight a s s es s many more f a c t o r s r ega rd ing o ne ' s p as th i s t o r y w it h an i n d i v i d u a l , s uc h a s (1) t h e number oft r a n s a c t i o n s one h a s had wi t h t h a t i n d i v i d u a l i n t h e p a s t , ( 2 )how he behaved i n those t ra ns ac t i on s , ( 3 ) t h e s i z e of p a y o f f s t o


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b oth p a r t i e s i n p re v io u s t r a n s a c t i o n s , ( 4 ) whether h i s t endencyt o c h e a t v a r i e d w i th t h e s i z e of t h e p ay of f i n vo l ve d , ( 5 ) whethert h e c o n d i t i o n s g o ve r ni ng h i s te nd en cy t o c h e a t h av e been s h i f t i n gover t ime, ( 6 ) h i s ( r e l a t i v e ) a g g r e ss i ve f o r m i da b i l it y , ( 7 ) howl i k e l y o n e i s t o m eet t h a t i n d i v i d u a l i n t h e f u t u r e ( e. g. , one ofyou i s moving away or ld ke ly t o di e so on ), and ( 8 ) whether one ofyou a c c e p te d a p a s t b e n e f i t b u t h a s n o t r e c i p r o c a t e d y e t .

A d e c i s i o n p ro c e du r e t h a t u se d s u c h d a t a , c u r r e n t b e h a v i o r a lc u e s, * a nd t h e p ay of f m a t r ix f o r t h e c u r r e n t i n t e r a c t i o n t ocompute t h e c o n d i t i o n a l p r o b a b i l i t y t h a t o n e ' s p a r t n e r w i l lcoope ra t e , m ight be be t t e r adap t ed t o t h e com pl ex i t i e s ofexchange i n na t u re . ** I f s o , t he n t h e need t o t a k e s uc h f a c t o r si n t o accoun t ha s i m p l i ca t i ons r e ga rd i ng t h e o rga n i za t i o n o f humanmemory. In fo rm a ti on abou t one ' s h i s t o r y of i n t e ra c t i o n w i t h ap a r t i c u l a r p e r so n o ug ht t o be "f i l e d " w i th t h a t p e r so n , andac t i v a t ed qu i ck l y and e f fo r t l e s s l y when an oppo r t un i t y f o r exchangew i t h t h a t pe r son a r i s e s . When t h e payof f m a t r i x o f t he cu r r en t

* For example, my f a c i a l e x p r es s io n m ig ht t i p o f f my i n t e n t i o n t ochea t you . A l l e l s e e q u a l , a p e r s o n ' s " l i k e a b i l i t y n s ho u ld be af u n c t i o n of h i s o r h er - te n de n cy t o r e c i p r o c a t e , and cues t ha tsugge s t "good in t e n t io ns " ought t o be judged more l i k ea b le (e . g .,sn ee r s and agg re s s i ve scow ls do no t sugges t good i n t e n t ) .A lth ou gh o t h e r e x p l a n a t i o n s a r e p o s s i b l e , i t i s i n t e r e s t i n g t h a tpeop l e remember u n fam i l i a r f a ce s be t t e r w hen, du r i ng i n i t i a li n s p e c t i o n , t he y a r e a s ke d t o judge t h e p e r s o n' s l i k e a b i l i t y th a nwhen t hey a r e a sked t o a s s i gn sex (Ca rey & Diamond, 1980).** An alg or i th m was su bmit ted t o Axel rod 's computer tournamentt h a t computed t h e c o n d i t i o n a l p r o b a b i l i t y t h a t a n i n t e r a c t a n twould coo per a te based on whether th a t i nd iv idu a l had coopera te do r de f ec t e d i n pa s t i n t e ra c t i o ns (REVISED DOWNING) . I t coope ra t edon l y when t h i s cond i t i ona l p ro ba b i l i t y was g r ea t e r t han 50%(random). I t s downfa l l was tha t it d i d no t d i scoun t pa s t behav i orr e l a t i v e t o p r e s e n t b eh av io r. T h er e fo r e, it was ex pl oi te d byc e r t a i n p rog rams wh ich became more l i ke l y t o chea t i n l a t e ri n t e r a c t i o n s . I n a s e ns e, i t f a i l e d b ec au se it assumed t h a tc o m p e t it o r p ro gr am s h ad s t a t i c " p e r s o n a l i t i e s . "


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i n t e r a c t i o n i s such t ha t you w i l l l o s e a g r e at d e a l i f I c h e a tyou, th en more of our p as t exchange h i s t o r y should becomeac ce ss ib le than fo r t r i v i a l exchanges . When you be l ie ve I haveche at ed you i n a major way, t he r e should be a fl oo d of memoriesabout your pas t h is tory with me: you must de ci de whether it i sw or th y ou r w h i l e t o c o n t i nu e o ur r e l a t i o n s h i p . I n a d d i t i o n , t h i sin ormation w i l l h el p you ne go t i at e wi'th me i f you choose t ocon ti nue our r el at io ns hi p: You can communicate how l ar g e a co st Ihave in f l i c t e d on you now and i n th e pas t (so I can make amendsi f I want t o con t inue th e re la t i on sh ip ) , t e l l me how c lose youcame t o end ing our re l a t io ns h i p ( i . e . , ca te gor iz i ng me a s apermanent de f ec t or ) , convince me th a t I have become increasinglyun t ru stwor thy, th re a t e n t o ru in my re pu ta t ion by t e l l i n g o t he r sabout my pa st t r ans gr ess io ns , and so on.

The a c t iva t i on of pa s t s i tu a t io ns i n which I have cheatedyou may, i n tu rn , ac t i va te o t he r* a f f ec t i ve mechanisms t h a tcommunicate cost/benef it in ormation: th ey may c ause you t o c r y ,t u r n your back on me, scream a t me, h i t me. The e x t e n t andna tu re of th e ove r t a s pe c t s of you r a f fe c t iv e re a c t i oncommunicates t o me your view of t h e ex te nt of my wrong doing:whether you view it a s s e r io us enough t o r e qu i re r e s t i tu t i on , howmuch i s re qu ir ed and how soon, whether you in t en d t o cu t me of fi f I d e f e c t aga in . Emotion communication can be viewed a s oneway ind iv id ua ls communicate co st , b en ef i t s , and beha vior a l

* I say "o t hern because I se e no pr i nc ip le d way of drawing adi vi di ng l i n e between emotion and cognit ion. The f l ood ofmemories you e xp er ie nc e when I betray you i s a s much a p a r tof your "emotional re act io n" a s your tur ni ng r ed and punching meout (s ee Tooby, i n pr ess ; Tooby & Cosmides, i n p r epa ra t i on) .i n t e n t o n s t o o t h e r s i n n e g o t i a t i v e s i t u a t i o n s ( s e e Cosmides, 1 98 3).


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P r o p o s i t i o n 3. One must be ab l e t o communicate one ' s val ues t oo t h e r s .To engage i n an exchange wit h you, I must know what you

want. Although lang uag e i s ce r t a i n l y a u se fu l means fo rcommunicat ing what one va lu es , no n- l i ng ui s t ic organisms can a l s oengage i n s oc i a l exchange -- however, th e range of i te ms the y canexchange i s ne ce ss ar i l y more l i mi te d. For example, chimpsr e c r u i t s u p p or t f ro m o t h e r s i n a g g r es e i v e e nc o u nt e r s, andf r eq uen t l y form long- te rm co a l i t i o na l r e l a t i ons h ip s . Theseco a l i t i o ns a r e so c i a l exchanges i n which t he exchanged " i tem" i sm utu al a i d i n f i g h t s . A chimp under a t t ac k bares i t s t e e t h ,emi ts a f e a r scream, looks a t t he ind iv i dua l from whom it wantssuppor t , and ho lds ou t i t s hand, palm up, toward t h a t ind iv id ua l .I f t he a t t a c ked chimp r ece ives t h e r eques t ed suppor t , i t sdemeanor changes radical ly: i t s ha i r s tan ds on end, i t emi t sagg r e s s i ve ba rks , and it charges i t s opponent -- looking over i t ss h ou l de r f r e q u e n t l y t o s e e i f i t s suppor t e r i s s t i l l with i t . I fthe ch imp does no t rece ive suppor t , i t cont inues cower ing wi thh a i r f l a t a nd t e e t h b ar ed , s cr ea mi ng and h ol d i ng o u t i t s hand t os o l i c i t s up po rt .

One a l s o mus t be ab le t o communicate d i s sa t i s f ac t i o n wi th adef ec t or . Th is a l so can be done wi thout language , a s i s v i v i d l yi l l u s t r a t e d by an in t e r a c t io n between Pu i s t and Lu i t , two chimpsi n th e Arnhem chimp colony i n th e Nether lands. Pu is t and L u i thad a l o ng - st a nd i ng c o a l i t i o n a l r e l a t i o n s h i p : P u i s t h ad a l o n ghi s t o ry of a i d i ng Lu i t whenever he a t ta cke d or was under a t t ac k ,and L u i t ha d a l o n g h i s t o r y o f e x te n di n g s i m i l a r a i d t o P u i s t .

Th i s happened once a f t e r Pu i s t had suppor t ed Lu i t i n chas ingNikkie [another chimp]. When Nikkie l a t e r dis pla ye d[ a g gr e s si v e ly ] a t P u i s t s he t u r n ed t o L u i t a nd h e l d o u t h e r


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hand t o him i n sea r ch of su ppor t . Lu i t , however, d i dn o t h i n g t o p r o t e c t h e r a g a i n s t N i k k i e ' s a t t a c k . Im me di at el yP u i s t t u r n e d on L u i t , b a r k i n g f u r i o u s l y , chased him a cr os sth e enc lo sur e and even h i t him. (de Waal, 1982, p. 2 0 7 )The communi ca ti on of d es i r e s , e n t i t l em en t s , and un f u l f i l l e do b l i g a t i o n s i s p o s s i b l e w i t h o u t la n gu a ge , g i v en t h a t t h ecommunicators a r e both programmed t o unde r s t and t h e s i gn al s . I tr e q u i r e s t h a t a g e s t u r a l / r e f e r e n t i a l s ys te m be s h a r e d by t h ep o t e n t i a l c o o p e r a t o r s .

A co gn i t i ve syst em t h a t can ena b l e t h e communicat ion ofd e s i r e s r e qu i r es more than the development of a few s ig ns . Thes i g ns mus t be coup l ed w i t h a r e f e r e n t i a l sys tem. I f I want t oexchange an axe fo r s omething, how do I i n d i c a t e w hat I want?L e t ' s s ay I po i n t t o t he pea r you a r e ho l d i ng i n your hand . Whatam I r e f e r i n g t o by p o i n t i n g t o t h e p e a r? Do I want tha tp a r t i c u l a r p e a r? Any p ea r a t a l l ? F i v e b u s h e l s o f p e a r s ? Af r u i t o f some k i nd , n o t n e c e s s a r i l y a p e a r? To be l e d t o t h es i t e wher e you f ound such n i ce pea r s? Do I want you t o hold abranch- ladder so I can c l i mb i n t o a t r e e which has pea r s ? Or at r e e w i t h some o t h e r k i nd o f f r u i t ? Do I want t o u s e my a xe t oco r e t h e pea r , i n exchange f o r ha l f t he pea r ? And so on.

The ambi gu it y of r e f e r en ce i n t h e absence of a sha r edr e f e r e n t i a l sy st em i s no mere ph i los oph ica l puz z le ( e .g ., Quine ,1969; Glei tman & Wanner, 19 82 ). For example, i t i s n o t c l e a rt h a t t h e i n f l i c t i o n of p a i n , i n t h e a bs en ce of a s h ar e dr e f e r e n t i a l f ramework, coul d communicate what it i s t h a t t hei n d i v i d u a l i n f l i c t i n g t h e p a i n wants t h e o t h e r i n d i v i d u a l t o s t o pdo ing . The d i f f i c u l t y of communi ca ti ng de s i r e s i n t he absence ofa sha r ed sys tem of r e f e r en ce i s i l l u s t r a t e d by c e r t a i n"communica tion gap sn t h a t occur between two d i f f e re n t , bu t


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c lo se l y re l a t ed , spec i es of baboons : hamadryas baboons andsavannah baboons.

A mal e hamadryas baboon ac q ui r es a "harem" of fe ma le s bykidnapping ju ve ni le females from oth er tro ops . He le ad s them t owater h o l e s and f e e d i n g gr ou nd s t h a t a r e w id el y s c a t t e r e d i n t h einho s p i t a b le E th iopia n bad la nds. To keep a ki dnapped f ema le fromst ra yi ng , th e male bi t e s her whenever she wanders even a few f e e tfrom where he wants her . But how does t h e fema le know what t h i sb i t e r e f e r s t o , what it i s t h a t the male does not want her t o do?T h i s may seem l i k e a s t r a ig h t f orwa rd case of "narrowinghypoth eses" through cond iti oni ng. However, t he same her din gte ch ni qu e does no t work on a fem al e savannah baboon. Whenabducte d i n t o a hamadryas mal e ' s harem, t he hamadryas male t r i e st o ke ep he r i n l i ne by b i t i ng he r , t o no a va i l . The sa va nnahfemale never "get s" what i t i s he wants, and simply runs off.For male s, knowing t h a t one can con di ti on hamadryas fem ale s byb i t i ng them a ppe a rs t o be no more " imp l i c i t i n the s i tu a t io n"t ha n knowing what a b i t e means. Savannah-hamadryas hy br i d maleswho l i v e among hamadryas baboons ca nno t keep a harem -- t h ehyb r id male ne ve r " f igu r e s ou t " t h a t it can herd females throughbi t ing (Hrdy , 1981) .

Apparen tly, th e l ea r ni ng mechanisms of hamadryas andsavannah baboons in cl ude di f f er e nt re f er en t i a l sys tems. Hamadryasmales and females both "know" t h a t a bi t e means " st ay with th eherd"; savannah baboons do not . The a b i l i t y t o smile, hug, ori n f l i c t p ai n i s not enough. A g e s t u r a l s ys tem f o r i n d i c a t i n gp r e f e r e n c e t h a t i s n o t c o g n i t i v e l y co up le d t o a r e f e r e n t i a lsystem would be i nacc ura t e a t be s t , and imposs ib le a t wors t .


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The ge s t u r a l / r e f e r en t i a l system t h a t a l lows members o f non-l i n g u i s t i c s p e c ie s t o s i g n a l c o s t s , b e n e f i t s , and be h av i or a li n t e n t i o n s t o c o n s p e c i f i c s c an be t h ou g ht o f a s a n em ot io ncommunica tion sys tem. Indeed, e th o l og is t s have t ra d i t i o na l l ycons idered such s i gn a l in g th e pr imary func t i on of emot iona le x p r e s s i o n , s t u d y i n g inke n t io n movements, co ur t s h i p dances ,a g o n i s t i c d i s p l a y s , and a g g r e s s i v e i n t e r a c t i o n s i n mammals,b i rd s , r e p t i l e s , f i s h , and in se c t s . L ike modern nonhumanp r im a te s , o ur p r e l i n g u i s t i c ho minid a n c e s to r s u nd ou bted ly hadsuch a system and used it t o communicate abo ut s o c i a l exchange.For example , t o t h i s day , humans a l l over th e g lobe sha re thesame f a c i a l expr ess ion s of emotion (Eibl-Eibesf e l d t , 1975; Ekman,1982); we even s har e many of the se f a c i a l exp ress ions wi thnonhuman pr im at es ( J o l l y , 1972 , pp. 158-159). The same i s t r u ef o r c e r t a i n a u d i to r y s i g na l s , l i k e screaming and cr yi ng (Eib l-E i b e s f e l d t , 1 9 7 5 ) . I can th in k of no reason why th e appeara nceof l anguage would cause t h i s more anc i en t sys tem to be se le c t edo u t . Mo reov er, t o t h e e x t e n t t h a t s uc h s i g n a l s a r e u n i v e r s a l l yshared , they have some in te re s t i ng p rop er t i e s which spoken1 nguage lacks :1. Because they a r e un i ver sa l ly shared , emotion s i g na l s can berecognized by anyone. By a i d i ng " t ra ns la t i on n , such s i g na lsexpand t h e r an ge of p o s s i b l e i n t e r a c t a n t s t o i n d i v i d u a l s whospeak a d i f f e r en t l anguage and ind i v i du a l s who cannot y e tspeak a language ( smal l ch i l d r en ) .2. Emotion s i g n a l s ca n fu n c t i o n l i k e i n t e r s u b j e c t i v e r u l e r s ,

p e r m i t t i n g a n o b se r ve r t o s c a l e t h e v a l u e s of t h e p e rs one m i t t i n g t h e s i g n a l : A v e r y l o u d s cr ea m in d i c a t e s a g r e a t e rco s t t o t he screamer than a modera te ly loud scream. S i gn a l sl i k e sc reams , smi l es , and t rembles a r e "ana log": The louderth e s cr ea m, t h e wide r t h e s m i l e , t h e more n o t i c e a b l e t h et r e m b le -- th e more s t r on gl y th e person can be presumed t of e e l a bo u t t h e s i t u a t i o n c a u si n g h er t o sc re am , s m i l e o rt remble . Words do not provid e such convenie nt r u l e r s ,p r e c i s e l y b ec au se t h e y a r e a r b i t r a r y and d i s c r e t e sym bols.


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V er ba l e x p r es s i o ns i n d i c a t i n g s i z e of c o s t o r b e n e f i t a r e more" di gi t a l " : One might reasonably use "very much" t o desc ri beth e deg re e of one ' s de s i re i n both the s e s e n te nc es : "I wantvery much fo r my ch i l d ' s cancer t o go i n t o remiss ion" and " Iwant t h a t app le very muchn -- y e t i n t h e s e two c a s e s t h edegree of d es i r e i s v a s t l y d i f f e r e n t .3 . Emotion s i g n a l s a l low th e i nc id en ta l communication of val uest o po te n t i a l in t e r a c t a n t s . By obse rving you r e mo tiona lr e a c t i o n s t o v ar i o u s s i t u a t i o n s , even thoug h t he y a r e n otd i r e ct e d a t m e , I can l e a r n what you val ue, and hence what s o r tof e xc ha nge you a r e l i ke ly t o a g re e t o ( s e e P ropos i t ion 4 ) .T h e v e r b a l a l t e r n a t i v e i s a p r oc e ss a k i n t o w r i t i n g t o S an t aClaus : Rec i t i ng , o r pub l ic ly post i r ig a long l i s t s t a t i n gone ' s p re fe re nc e h iera rchy .. .wi th pe r i od ic upda te s l*

However, th e very pr op er t i es t h a t make a na tu r a l language apoor medium f o r communicating i n t e n s i t y of a f f e c t make it anexc e l le n t system fo r ind i ca t in g " i tems" of exchange. The va r i e t yof "ite ms" t h a t can be exchanged i s s e v e re l y l i m i t e d i n a s p e c i e st h a t uses on ly emotion s i gna ls . Pr imates appear to exchangef i g h t f o r f i g h t , f i g h t f o r s ex , s ex f o r s e x , f ood f o r f o od ,f i g h t , or sex, groom f or groom, groom f or f i g h t , food, orsex...and no t to o much e l se . The use of language does no t, ofcour se , e l i mi nat e th e problem of ambiguous ref eren ce. I n th eabsence of a shar ed re f e r e n t i a l semantics , knowing what a wordr e f e r s t o i s no l e s s problematic than knowing what a gesturer e f e r s to.** But a na t u r a l l anguage pe rmi t s a po t e n t i a l l yi n f i n i t e number of a r b i t r a r y , d i s c r i mina b le s ymbo ls t o be

* A c t u al l y , a S a n t a ' s l i s t s t a t i ng th a t you want X , Y, and Z i sno t s u f f i c i e n t . Your p re fe re nc e s -- inc lud i ng i t ems you a l readyhave -- would have t o be h i e r a r ch ic a l ly o rdered us ing some so r tof i n t e r v a l s c a l e o r i n d i f f e r e n c e c u rv e s, b ec au se t h e s a l i e n ti s s u e i s : What would you be wi l l i ng t o gi ve up i n order t o ge tX,Y , and Z ?** Th i s problem has prompted developmental psycholing ui s t s t opos i t tha t c h i ld re n ha ve inna te ly s pe c i f i e d "hypo the s e s " a bou twhat s o r t s of e n t i t i e s a r e l i k e l y t o ha ve words a t t a c h e d t o them.When coupled with a r t i c ul a t ed models of t he world , t h i shypo the s i s + model sys tem amounts t o a r ef er e nt i a l semantics(Gleitman & Wanner, 1982) .


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a t t a c h e d t o a p o t e n t i a l l y i n f i n i t e number o f d i s c r i m i n a b l ec l a s s e s o r e n t i t i e s . A s new si t u a t i o n s a r i s e , new words can beo p p o r t u n i s t i c a l l y c r e a t e d t o r e f e r t o them. C on se qu en tl y,l anguage pe rm i t s a r ange and sp ec i f i c i t y of r e fe rence im poss ib lei n the pure ly ge s t u r a l syst ems of most p r ima tes .

Th is pro per ty of language opens th e va st realm of humana d a p t a t i o n s a s s o c i a t e d w i t h p l a nn i n g and t o o l- u s e t o s o c i a lexchange. Tool technology co nt in ua l l y changes, * with new toolsbein g inv en ted co ns ta nt ly . New tech nolo gies en abl e new andc o n s t a n t l y c ha ng in g o p p o r t u n i t i e s f o r c o o rd i n a te d , c o o p e r a t i v ebehaviors which can themselves become "items" of exchange. Greatbe ne f i t s can be had by exchanging too l s and by pa r t i c ip a t in g i nt h e c omplex and o p p o r t u n i s t i c a l l y s h i f t i n g c o o p er a t i v ee n t e r p r i s e s t h e s e a l l o w -- U if t o o l s m a v i o r a lxout i - m k amed. The expanded power of re fe re nc e t h a tlanguage aff or ds i n s o c i a l exchange may have been one fac to rs e l e c t i n g f o r i t s e v o l u t i o n . I t i s n o t c l e a r t h a t a ny b ut t h esimp1e s t t oo l -us ing coopera t ive en te r p r i s e s cou ld be accom pl i shedw i t h a n o n - l i n g u i s t i c g e s t u r a l s ys te m -- r o u ti n e s l i k e t h ec hi mp s' l a d d er e x p e d i t i o n , t h a t a r e di s c o ve r e d q u i t e p u b l i c l yi n th e con tex t of an em ot iona lly s a l i e n t even t, *+ and don ' tr eq u i r e long pe r iods o f p l ann ing .

* A t l e a s t f o r Homo m w i en s m i e n s . Homo e r e c t u s ' t o o l k i ts tay ed id en t i ca l over a wide range of d i f fe re nt envi ronments --f rom Asia t o Afr ic a -- fo r over 1.5 mi l l i on yea rs (Pilbeam,pers onal communicat ion) . Of course , t h i s obse rvat ion ap pl ie so nl y t o t o o l s t h a t a r e r e co gn iz ab le a s s uch i n t h e f o s s i l r ec or d.For example, a branch used a s a la dd er would no t show up i n th ef o s s i l r e c o r d .** The Arnhem chimps disc ov ere d th e la dd er t r i c k when onescreaming chimp, f l e e i n g from a v e r y p u b l i c a t t ac k , bounded up abroken b ranch t h a t happened t o be r e s t i ng aga ins t a t r ee .


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The e v o l u t i o n o f l a n gu a ge do es n o t o b v i a t e t h e a b i l i t y t ocommunicate co s t / be ne f i t i n f o r ma t i on t h rough emoti on s i g na l s . I nf a c t , th e more i t e ms th a t members of a s pe c i es can name andexchange , and t h e more t h e i n s t r ume n t a l va l ue o f t h es e i t emsv a r i e s between in di v i du al s and over t ime, th e more one needs an" i tem- independen tn y e t un i ve r s a l l y under s t ood s yst em f o rcom mun icat ing how much one v a lu e s an i t e m .

Because t h e va r i e t y of items exchanged by non l i ng ui s t i cp r i m a t e s i s s o l i m i t e d , each i tem cou l d , i n t heo r y , have a un iquec o s t / b e n e f i t w e i gh t in g a s s o c i a t e d w i th it t h a t i s sh ar ed by mosto t he r members of th e spe c i es ( e .g . , t en grooms dese rves one f i g h t ,a s eas on of p r o t e c t i on by a m al e d e s e r v e s e x c l u s i v e s e x u a l a c c e s sa t t h e h e i g h t of e s t r u s , e t c . ) . In o th er words , each i t em couldhave a preprogrammed, un iv er sa l l y acknowledged, "exchange ra te ."

But th e r e can be no preprogrammed, un iv er sa l l y acknowledged," ex ch an ge r a t e w f o r a c o n s t a n t l y c ha ng in g a r r a y of t o o l s an dcoo rdi na t ed be hav iora l rout in es . Language combined wi th emot ions i gn a l i ng a f f o rd s a uniquely powerf u l communica tive system fo rs o c i a l e xc ha ng e i n a p la n n i n g , t o o l u s i ng , and o p p o r t u n i s t i c a l l yc o o p e r a t i v e , s p e c i e s . A wi de va r i e t y o f items can be p r ec i s e l ys p e c i f i e d th ro u gh l an gu ag e, and t h e i r r e l a t i v e v a l u e t o a nin di v i du al can be s imul tan eous ly communica ted -- e i t h e ri n c i d e n t a l l y * o r i n t e n t i o n a l l y -- v i a emoti on s i g na l s . I ndeed,t h e r e i s r ud imen ta r y ev i dence s ugges t i ng t h a t some as pec t s o f t he* Because t h e i n c i de n t a l communication of co s t / be ne f i ti n f o r ma t i on i s i mpor t an t ( s ee P r op os i t i on 4 ) , one might predic tt h a t , a l l e l se e q u al , i n d i v i d u a l s a r e more l i k e l y t o emit emotions i g n a l s i n t h e p re s e nc e ( o r s u sp e c te d p r e se n c e ) of p o t e n t i a lr e c i pr oc a t or s than when a lone . S i mi la r ly , they should be morel i k e l y t o s u p p r e s s e mo tio n s i g n a l s i n t h e p r e se n ce of p o t e n t i a la g g r e s s o r s -- v a l u e i n f o r m a t i o n h e l p s a g g r e s s o r s ; i t t e l l s themwha t t hey s hou ld t h r ea t en t o k i l l , d e s t r o y , o r p r e v e n t .


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ac ou st ic exp res s ion of emotion i n humans have been i nt eg ra t edi n to ou r s pe c ie s- s pe ci f i c language capa c i t y i n ways t h a tf a c i l i t a t e th e communcation of va lues and in te n t io ns (Cosmides , 1983) .

P r o p o s i t i o n 4 . One must be ab l e t o model t he va lue s of o t heri n d i v i d u a l s .I n some ways, Pr opo si ti on 4 i s j us t t he f l i p s i d e of

P ropos i t ion 3: One must have a co gn it iv e system capa bl e ofdecoding communicat ions of the so r t desc r ibed i n Propos i t i on 3 .In a dd i t ion t o t h i s , however, one ought t o ha ve l e a rn ingmechanisms t ha t a r e s pe c i a l i z e d fo r p ic k ing up inc i de n ta lino rmat ion a bou t the value s of p o te n t i a l in te r a c t a n t s -- f o rdoing "marketing resear ch". I n order t o propose an exchange f ormutual benefit, one must have some notion of what kind of "item"t h e o t h e r i n d i v i d u a l i s l i ke l y t o va lue . The ind iv i dua l who i swell -equ ipped t o do "marke t ing resea rchn on po t en t i a li n t e r a c t a n t s w i l l be a b l e t o sugges t f a r more exchanges than th ei n d i v i d u a l who w a i t s f o r p o t e n t i a l i n t e r a c t a n t s t o i n t e n t i o n a l l ya nnounc e the i r p re fe re nc e h ie ra rc h ie s .

Because e mo tion s i gn a l s f l a g c os t /be n e f i t in fo rmat ion , theys ho ul d a u to m at i c al l y r e c r u i t a t t e n t i o n and be d i f f i c u l t t oignore. An ea r- sp l i t t in g scream should be more d i f f i c u l t t oigno re tha n an e qua l ly loud t r a i n wh is t l e ; s o s obb ing f rom thenext room should be harder t o igno re tha n th e loud honk of a ca rhorn ou ts ide . A broad s mi le s hould

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