On the evolutionary origins of altruistic behavior: can selection at the individual level be enough

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On the evolutionary origins of altruistic behavior: can selection at the individual level be enough

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1. 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

2. Cooperation: the basis of human societies Occurs between genetically unrelated individuals

3. Cooperation: the basis of human societies Shows high division of labor

4. Cooperation: the basis of human societies Valid for large scale organizations…

5. Cooperation: the basis of human societies Valid for large scale organizations…

6. Cooperation: the basis of human societies

7. Altruism: key to cooperation

8. How did altruism arise?

9. Altruism is an evolutionary puzzle

10. Group selection? Cultural evolution?

11. Answers to the puzzle… Kin cooperation (Hamilton, 1964) common to animals and humans alike Reciprocal altruism in repeated interactions (Trivers, 1973; Axelrod & Hamilton, 1981) primates, specially humans Indirect reciprocity (reputation gain) (Nowak & Sigmund, 1998) primates, specially humans

12. … but only partial! Strong reciprocity (Gintis, 2000; Fehr, Fischbacher & Gächter, 2002) 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

13. Experiments

14. Altruistic punishment: the Ultimatum Game (Güth, Schmittberger & Schwarze, 1982)

15. Experimental results Proposers offer substantial amounts (50% is a typical modal offer) Responders reject offers below 25% with high probability Universal behavior throughout the world Large degree of variability of offers among societies (26 - 58%)

16. Experimental results

17. Experimental results

18. Altruistic rewarding (Fehr, Kirchsteiger & Riedl, 1993)

19. Experimental results More than 50% of trustees transfer money back, the more the higher the truster’s transfer Players often exchange M Universal behaviour throughout the world

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

21. Strong reciprocity 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 Strong reciprocity seems a crucial element to ensure cooperation in human societies

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

23. Is group selection unavoidable?

24. Our model: setup

25. Our model: game step

26. Our model: every s steps

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

28. Small populations

29. Slow selection

30. Large populations

31. Slow selection

32. Why are reciprocators selected? 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 Mutations are small (darwinian), so newborn altruistic agents are just a little more altruistic A quantitative description is still lacking

33. Two thresholds (s small)

34. Two thresholds (s middle)

35. Two thresholds (s large)

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

37. Mutation rate effect

38. Related results

39. Related results

40. Related results

41. Related results

42. Related results

43. Discussion 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 Fluctuations may keep players with smaller ‘mean-field’ fitness alive

44. Further discussion 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)

45. 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

46. Adam Smith on altruism

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