1. 8th Granada Seminar – Modelling Complex Social Systems
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
Anomaly in the animal world:
Occurs between genetically unrelated individuals

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

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

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

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

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

8. How did altruism arise?
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)

9. Altruism is an evolutionary puzzle

10. Group selection? Cultural evolution?
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)

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)
None true altruism: individual benefits in the long run

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
Hammerstein (ed.), Genetic and cultural evolution of cooperation (Dahlem Workshop Report 90, MIT, 2003)

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

14. Experimental results Responder’s optimal strategy: accept anything
Proposer’s optimal strategy: offer the minimum
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 ( %)

15. 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)

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

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

18. Experimental results Both players gain the maximum by exchanging M
The trustee gains more by returning nothing
So truster’s best strategy is to transfer nothing
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
Homo economicus vs. Homo reciprocans
Review in Fehr & Fischbacher, Nature 425, 785 (2003)

19. Strong reciprocity Explanations for altruistic behaviour:
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

20. Strong reciprocity Importance of 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

21. Strong reciprocity Evolutionary origins of 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

22. Is group selection unavoidable?
Is evolutionary game theory correct?

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

24. Our model: game step tp tr tp tr ≥ < ...... N players tp tr fp fr
proposer
responder tp fp tr fr
+M-tp
+tp
tp tr
<
tp tr ≥

25. Our model: every s steps
......
N players
new player
minimum fitness
maximum fitness
tmin
fmin
t’max
fmax
tmax
fmax +1 -1
(prob.=1/3)
mutation: t’max= tmax

26. 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

27. Small populations
N =1000, 107 games, s =1, uniform initial condition

28. Slow selection
N =1000, 108 games, s =104, ti =1 initial condition

29. Large populations
N =106, 2x107 games, s =1, uniform initial condition

30. Slow selection
N =105, 1.28x1011 games, s =104, ti =1 initial condition

31. 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

32. Two thresholds (s small)
N =1000, 106 games, s =1, uniform initial condition
accept
offer

33. Two thresholds (s middle)
N =1000, 108 games, s =1000, ti = ai =1 initial condition
accept
offer

34. Two thresholds (s large)
N =1000, 109 games, s =105, ti = ai =1 initial condition
accept
offer

35. 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

36. Mutation rate effect
N =10000, 108 games, s =1, ti = 1 initial condition

37. Related results

38. Related results

39. Related results Stag-hunt game: S R
C. P. Roca, J. A. Cuesta & A. S., in progress (2005)
Stag-hunt game:
Two equilibria, S and R
Round-robin gaming: S or R selected from x0 (x fraction of R)
s games between death-birth
S R

40. Related results

41. Related results
Random death Death proportional
to fitness

42. 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

43. 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)

44. 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
A new perspective in evolutionary game theory: more general dynamics, dictated by the specific application

45. 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)