Main ThemesReciprocityPrisoner's DilemmaEvolution of Human Reciprocity?Soci111 – Human SocietiesModule 5 – Cooperation & ReciprocityFrançois NielsenUniversity of North CarolinaChapel HillSeptember 10, 2013OutlineMain ThemesReciprocityPrisoner’s DilemmaEvolution of Human Reciprocity?Main ThemesÉHow the logic of the Prisoner’s Dilemma seems to preclude theevolution of cooperation by natural selectionÉHow the logic of the iterated Prisoner’s Dilemma and the TitFor Tat strategy allows cooperation to evolveÉRobert Trivers’ theory of the evolution of innate humanemotional propensities facilitating cooperation based on TitFor TatÉPsychological experiments by John Tooby and Leda Cosmidessupporting existence of a “cheater detection” module in thehuman psyche implementing the Tit For Tat strategyReciprocityCooperation in NatureÉExamples of cooperation in natureÉfood sharing (by regurgitation) among vampire batsÉcleaning fish and its hostÉ“live and let live” custom of trench warfare during World War IÉdolphins helping human swimmers in distressÉHow can cooperation between unrelated individuals(sometimes of different species) evolve?ReciprocityContingent ReciprocityÉRobert Trivers’s theory of reciprocal altruism (or contingentreciprocity) explains howÉin reciprocal altruism, there isÉtrading of beneficial acts, so thatÉover period of time both participants enjoy a net gain in fitnessÉe.g., vampire bats sharing foodÉBUT: major theoretical difficulty:Éindividual always tempted to cheat by not reciprocating (theselfish strategy)Éso cooperation should not evolve!ReciprocityContingent ReciprocityÉ← Robert Trivers & Huey NewtonÉTrivers’ insight:Éwhen interactions among sameindividuals areÉfrequentÉrepeatedÉindividuals may “punish” cheaterby withholding future cooperationÉthis is called a tit-for-tat strategy(TFT)Écooperation can evolve based onTFTPrisoner’s DilemmaBasic ModelÉPrisoner’s Dilemma (PD) in game theory is based on animaginary scenario:ÉTwo accomplices in a crime are caughtÉThey are placed in separate cells and they cannotcommunicate with each otherÉThe public attorney negotiates with each one to turn evidenceagainst the other (to defect)ÉRefusing to turn evidence against one’s accomplice is tocooperateÉThe payoff matrix shows the benefit resulting from differentcourses of actionPrisoner’s DilemmaBasic ModelTable: Prisoner’s Dilemma (PD)D CD 2 4C 1 3Ématrix entry shows payoff ofstrategy for prisoner in rowÉgiven strategy adopted byprisoner in columnÉstrategies areÉC, CooperateÉD, DefectÉin basic PD model the dominantevolutionary strategy is D(defection)Éthis suggests cooperationcannot evolve by naturalselectionPrisoner’s DilemmaIterated PD & Tit For TatTable: Iterated PDD TFTD 20 22TFT 19 30Ématrix entry shows totalpayoff for PD repeated 10timesÉstrategies areÉD, DefectÉTFT, Tit For Tat:Éat first cooperateÉthen repeat move (C orD) of protagonistÉin iterated PD TFT strategyis dominantÉ⇒ cooperation with TFT canevolve when sameindividuals interactrepeatedlyPrisoner’s DilemmaIterated PD & Tit For TatÉRobert Axelrod in The Evolution of CooperationÉorganizes computer tournament of iterated PDÉTFT turns out to be optimalÉagainst pure C, pure D, and itselfÉagainst many more complicated strategiesPrisoner’s DilemmaIterated PD & Tit For TatTable: Various meanings of cooperation & defectionActors Cooperation DefectionPrisoners Refusing to give ev-idence against theotherGive evidence againstthe otherBirds Remove ticks Refuse to removeticksVampire bats Regurgitate blood Refuse to regurgitatebloodHumans Do someone a favor Refuse to return thefavorSoldiers in trenchwarfareDo not shoot at en-emy during truceperiodShoot at enemy dur-ing truce periodEvolution of Human Reciprocity?Trivers’ TheoryÉRobert Trivers in SocialEvolutionÉevolution of innate capacityfor reciprocal altruism inhumans becauseÉlong life spanÉlow dispersal rateÉlife in small communitiesÉwhere everyone knowseach otherÉ⇒ repeated interactionsover long periodsÉ⇒ conditions forevolution of TFTEvolution of Human Reciprocity?Trivers’ TheoryÉHumans have evolved innate moral-emotional propensities toreact in adaptive ways to various contingencies related toreciprocal altruism, e.g.:Épropensity for friendshipÉcapacity for gratitude and sympathyÉpropensity for moralistic aggression against non-cooperationÉcapacity for guilt and reparative altruismÉsense of justiceEvolution of Human Reciprocity?Cosmides & Tooby ExperimentsLeda Cosmides, Steven Pinker, JohnTooby in 2005ÉEvolutionarypsychologists LedaCosmides & JohnToobyÉIs there an innatepsychological“module” to detectcheaters inreciprocalinteractions?Evolution of Human Reciprocity?Cosmides & Tooby Experimentspersongoing toBostonperson go-ing to Miamiperson tak-ing subwayperson tak-ing cabÉRule: “If a person goes to Boston, then he/she takes thesubway.”Ésubject shown 4 cardsÉeach card represents a personÉone side tells city where person goesÉother side tells mode of transportation person usesÉwhich card(s) must be turned over to detect violation of rule?1. person going to Boston2. person taking a cabEvolution of Human Reciprocity?Cosmides & Tooby Experimentspersongoing toBostonperson go-ing to Miamiperson tak-ing subwayperson tak-ing cabÉRule: “If a person goes to Boston, then he/she takes thesubway.”Ésubject shown 4 cardsÉeach card represents a personÉone side tells city where person goesÉother side tells mode of transportation person usesÉwhich card(s) must be turned over to detect violation of rule?1. person going to Boston2. person taking a cabEvolution of Human Reciprocity?Cosmides & Tooby Experimentspersondrinkingbeerpersondrinkingcoke25 years old 16 years oldÉRule: “If a person is drinking beer, then he/she must be over21 years old.”Ésubject shown 4 cardsÉeach card represents a personÉone side tells what the person is drinkingÉother side tells how old the person isÉwhich card(s) must be turned over to detect violation of rule?1. person drinking beer2. 16 years oldEvolution of Human Reciprocity?Cosmides & Tooby Experimentspersondrinkingbeerpersondrinkingcoke25 years old 16 years oldÉRule: “If a person is drinking beer, then he/she must be over21 years old.”Ésubject shown 4 cardsÉeach card represents a personÉone side tells what the person is drinkingÉother side tells how old the person isÉwhich card(s) must be turned over