On the evolutionary origins of altruistic behavior: can selection at the individual level be enough? José A. Cuesta & Anxo Sánchez GISC/Matemáticas Universidad Carlos III de Madrid 8th Granada Seminar – Modelling Complex Social Systems

Cooperation: the basis of human societies Occurs between genetically unrelated individuals Occurs between genetically unrelated individuals Anomaly in the animal world:

Cooperation: the basis of human societies Shows high division of labor Shows high division of labor Anomaly in the animal world:

Cooperation: the basis of human societies Valid for large scale organizations… Valid for large scale organizations… Anomaly in the animal world: …as well as hunter-gatherer groups

Cooperation: the basis of human societies Valid for large scale organizations… Valid for large scale organizations… Anomaly in the animal world: …or setting up nice events!

Cooperation: the basis of human societies Some animals form complex societies… …but their individuals are genetically related

Altruism: key to cooperation Altruism: fitness-reducing act that benefits others fitness-reducing act that benefits others Pure altruism is ruled out by natural selection acting on individuals á la Darwin

He who was ready to sacrifice his life (…), rather than betray his comrades, would often leave no offspring to inherit his noble nature… Therefore, it seems scarcely possible (…) that the number of men gifted with such virtues (…) would be increased by natural selection, that is, by the survival of the fittest. Charles Darwin (Descent of Man, 1871) How did altruism arise?

Altruism is an evolutionary puzzle

A man who was not impelled by any deep, instinctive feeling, to sacrifice his life for the good of others, yet was roused to such actions by a sense of glory, would by his example excite the same wish for glory in other men, and would strengthen by exercise the noble feeling of admiration. He might thus do far more good to his tribe than by begetting offsprings with a tendency to inherit his own high character. Charles Darwin (Descent of Man, 1871) Group selection? Cultural evolution?

Answers to the puzzle… Kin cooperation (Hamilton, 1964) Kin cooperation (Hamilton, 1964) common to animals and humans alike common to animals and humans alike Reciprocal altruism in repeated interactions (Trivers, 1973; Axelrod & Hamilton, 1981) Reciprocal altruism in repeated interactions (Trivers, 1973; Axelrod & Hamilton, 1981) primates, specially humans primates, specially humans Indirect reciprocity (reputation gain) (Nowak & Sigmund, 1998) Indirect reciprocity (reputation gain) (Nowak & Sigmund, 1998) primates, specially humans primates, specially humans None true altruism: individual benefits in the long run

… but only partial! Strong reciprocity Strong reciprocity (Gintis, 2000; Fehr, Fischbacher & Gächter, 2002) (Gintis, 2000; Fehr, Fischbacher & Gächter, 2002) typically human (primates?) typically human (primates?) –altruistic rewarding: predisposition to reward others for cooperative behavior –altruistic punishment: propensity to impose sanctions on non-cooperators Strong reciprocators bear the cost of altruistic acts even if they gain no benefits Strong reciprocators bear the cost of altruistic acts even if they gain no benefits Hammerstein (ed.), Genetic and cultural evolution of cooperation (Dahlem Workshop Report 90, MIT, 2003)

Experiments experimenter subject 1 subject month’s income

Altruistic punishment: the Ultimatum Game (Güth, Schmittberger & Schwarze, 1982) experimenter proposer responder M euros M-u u OK u M-u NO 0 0

Experimental results Proposers offer substantial amounts (50% is a typical modal offer) Proposers offer substantial amounts (50% is a typical modal offer) Responders reject offers below 25% with high probability Responders reject offers below 25% with high probability Universal behavior throughout the world Universal behavior throughout the world Large degree of variability of offers among societies ( %) Large degree of variability of offers among societies ( %) Proposer’s optimal strategy: offer the minimum Responder’s optimal strategy: accept anything Results:

Experimental results Extraordinary amount of data Camerer, Behavioral Game Theory (Princeton University Press, 2003) Henrich et al. (eds.), Foundations of Human Sociality : Economic Experiments and Ethnographic Evidence from Fifteen Small-Scale Societies (Oxford University Press, 2004)

Experimental results Paciotti et al., American Scientist 93, 58 (2005) Ultimatum game as an interpreting tool

Altruistic rewarding (Fehr, Kirchsteiger & Riedl, 1993) experimenter truster trustee M M -u +2u -v +2v

Experimental results More than 50% of trustees transfer money back, the more the higher the truster’s transfer More than 50% of trustees transfer money back, the more the higher the truster’s transfer Players often exchange M Players often exchange M Universal behaviour throughout the world Universal behaviour throughout the world Homo economicus vs. Homo reciprocans Review in Fehr & Fischbacher, Nature 425, 785 (2003) Both players gain the maximum by exchanging M The trustee gains more by returning nothing Results: So truster’s best strategy is to transfer nothing

Strong reciprocity Kin cooperation selfish genes Kin cooperation selfish genes Reciprocity tit-for-tat strategy (Axelrod’s experiments) Reciprocity tit-for-tat strategy (Axelrod’s experiments) Reputation gain altruism as sexual ornament (handicap principle) Reputation gain altruism as sexual ornament (handicap principle) Strong reciprocity sense of fairness requires “other-regarding” capabilities Strong reciprocity sense of fairness requires “other-regarding” capabilities Explanations for altruistic behaviour:

Strong reciprocity In experiments of public goods, the presence of a few cheaters quickly deteriorates cooperation In experiments of public goods, the presence of a few cheaters quickly deteriorates cooperation The introduction of a few altruistic punishers forces long term cooperation The introduction of a few altruistic punishers forces long term cooperation Strong reciprocity seems a crucial element to ensure cooperation in human societies Strong reciprocity seems a crucial element to ensure cooperation in human societies Importance of strong reciprocity:

Strong reciprocity Standard answer: cultural evolution through group selection Standard answer: cultural evolution through group selection Evidences in favour: Evidences in favour: –Strong variability between cultures in altruistic patterns –Increase of altruistic behaviour in children as they grow –Simulations of group selection models Evolutionary origins of strong reciprocity:

Is group selection unavoidable? Is evolutionary game theory correct?

Our model: setup N players player i t i : threshold (minimum share player i accepts) f i : fitness (accumulated capital) M monetary units (M=100) A.S. & J. A. Cuesta, J. Theor. Biol., in press (2005)

Our model: game step N players proposer responder tptpfpfptptpfpfp trtrfrfrtrtrfrfr t p t r ≥ +M-t p +t p t p t r <

Our model: every s steps N players minimum fitness maximum fitness t min f min t max f max new player t’ max f max mutation: t’ max = t max (prob.=1/3) +1

Implemented features Degree of altruism inherited from progenitor (with some error) Degree of altruism inherited from progenitor (with some error) No learning from experience No learning from experience No influence from other players (no culture) No influence from other players (no culture) No groups No groups Other-regarding behaviour with optimization (players offer the minimum they would accept) Other-regarding behaviour with optimization (players offer the minimum they would accept) Fitness (accumulated capital) is inherited Fitness (accumulated capital) is inherited

Small populations N =1000, 10 7 games, s =1, uniform initial condition

Slow selection N =1000, 10 8 games, s =10 4, t i =1 initial condition

Large populations N =10 6, 2x10 7 games, s =1, uniform initial condition

Slow selection N =10 5, 1.28x10 11 games, s =10 4, t i =1 initial condition

Why are reciprocators selected? Higher threshold implies lower payoffs but higher acceptance rates for agent’s own offers Higher threshold implies lower payoffs but higher acceptance rates for agent’s own offers The presence of altruistic punishers increases rejection of selfish agents’ offers The presence of altruistic punishers increases rejection of selfish agents’ offers Mutations are small (darwinian), so newborn altruistic agents are just a little more altruistic Mutations are small (darwinian), so newborn altruistic agents are just a little more altruistic A quantitative description is still lacking A quantitative description is still lacking

Two thresholds (s small) N =1000, 10 6 games, s =1, uniform initial condition acceptoffer

Two thresholds (s middle) N =1000, 10 8 games, s =1000, t i = a i =1 initial condition acceptoffer

Two thresholds (s large) N =1000, 10 9 games, s =10 5, t i = a i =1 initial condition acceptoffer

Discussion Altruistic punishment is not necessarily a losing strategy when there are many agents Altruistic punishment is not necessarily a losing strategy when there are many agents Thus it may be established by individual selection alone Thus it may be established by individual selection alone Results reproduce the observed variability Results reproduce the observed variability One or two levels: not important for the arising of altruistic punishment One or two levels: not important for the arising of altruistic punishment Mutation rate might be relevant Mutation rate might be relevant

Mutation rate effect N =10000, 10 8 games, s =1, t i = 1 initial condition

Related results

Stag-hunt game: Two equilibria, S and R Round-robin gaming: S or R selected from x 0 (x fraction of R) s games between death-birth S R C. P. Roca, J. A. Cuesta & A. S., in progress (2005)

Related results

Random death Death proportional to fitness

Discussion In general, evolutionary game theory studies a limit situation: s infinite! (every player plays every other one before selection) In general, evolutionary game theory studies a limit situation: s infinite! (every player plays every other one before selection) Number of games per player Poisson distributed Number of games per player Poisson distributed Fluctuations may keep players with smaller ‘mean-field’ fitness alive Fluctuations may keep players with smaller ‘mean-field’ fitness alive

Further discussion Thresholds observed in neural activity measurements (Sanfey et al., 2003) (suggest that this threshold is an inheritable feature) Thresholds observed in neural activity measurements (Sanfey et al., 2003) (suggest that this threshold is an inheritable feature) Correlation with other-regarding behavior: also in different primates (capuchin monkeys) (Brosnan & de Waal, 2003) Correlation with other-regarding behavior: also in different primates (capuchin monkeys) (Brosnan & de Waal, 2003)

Main conclusions Altruism (altruistic punishment, as implemented in the Ultimatum Game) need not be an evolutionary losing strategy, so it may have arisen through standard individual selection Altruism (altruistic punishment, as implemented in the Ultimatum Game) need not be an evolutionary losing strategy, so it may have arisen through standard individual selection A new perspective in evolutionary game theory: more general dynamics, dictated by the specific application A new perspective in evolutionary game theory: more general dynamics, dictated by the specific application

Adam Smith on altruism How selfish soever man may be supposed, there are evidently some principles in his nature, which interest him in the fortunes of others, and render their happiness necessary to him, though he derives nothing from it, except the pleasure of seeing it. Adam Smith (The Theory of Moral Sentiments, 1854, Chapter 1)