on the evolutionary origins of altruistic behavior: can selection at the individual level be enough?...

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

José A. Cuesta & Anxo José A. Cuesta & Anxo SánchezSánchez

GISC/Matemáticas GISC/Matemáticas Universidad Carlos III de MadridUniversidad Carlos III de Madrid

8th Granada Seminar – Modelling Complex Social Systems8th Granada Seminar – Modelling Complex Social Systems

Cooperation: the basis Cooperation: the basis of human societiesof human societies

Occurs between genetically unrelated Occurs between genetically unrelated individualsindividuals

Anomaly in the animal Anomaly in the animal world:world:


Shows high division of laborShows high division of labor



Valid for large scale organizations…Valid for large scale organizations…


……as well as hunter-gatherer groupsas well as hunter-gatherer groups


Valid for large scale organizations…Valid for large scale organizations…


……or setting up nice events!or setting up nice events!


Some animals form complex Some animals form complex societies…societies…

……but their individuals are genetically but their individuals are genetically relatedrelated

Altruism: key to Altruism: key to cooperationcooperation

Altruism:Altruism:

fitness-reducing act that benefits othersfitness-reducing act that benefits others

Pure altruism is ruled out by natural Pure altruism is ruled out by natural selection acting on individuals selection acting on individuals á la Darwiná 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?How did altruism arise?

Altruism is an evolutionary Altruism is an evolutionary puzzlepuzzle

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 Group selection? Cultural evolution?evolution?

Answers to the puzzle… Answers to the puzzle…

Kin cooperationKin cooperation (Hamilton, 1964)(Hamilton, 1964)

common to animals and humans alikecommon to animals and humans alike Reciprocal altruism in repeated Reciprocal altruism in repeated

interactionsinteractions (Trivers, 1973;(Trivers, 1973; Axelrod & Hamilton, Axelrod & Hamilton, 1981)1981)

primates, specially humansprimates, specially humans Indirect reciprocity (reputation gain)Indirect reciprocity (reputation gain)

(Nowak & Sigmund, 1998)(Nowak & Sigmund, 1998)

primates, specially humansprimates, specially humansNone true altruism: individual benefits in the None true altruism: individual benefits in the long runlong run

… … but only partial!but only partial!

Strong reciprocityStrong reciprocity (Gintis, 2000; Fehr, Fischbacher & Gächter, 2002)(Gintis, 2000; Fehr, Fischbacher & Gächter, 2002)

typically human (primates?)typically human (primates?)– altruistic rewarding:altruistic rewarding: predispositionpredisposition to to

reward others for cooperative behaviorreward others for cooperative behavior– altruistic punishment:altruistic punishment: propensitypropensity to to

impose sanctions on non-cooperatorsimpose sanctions on non-cooperators Strong reciprocators bear the cost of Strong reciprocators bear the cost of

altruistic actsaltruistic acts even if they gain no benefits even if they gain no benefits

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

ExperimentsExperiments

experimentexperimenterer

subject 1subject 1 subject 2subject 2

2-3 month’s income2-3 month’s income

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


proposeproposerr

respondresponderer

M M euroseuros

M-uM-u uuOKOK

uuM-uM-u

NNOO

00 00

Experimental resultsExperimental results

Proposers offer substantial amounts (50% Proposers offer substantial amounts (50% is a typical modal offer)is a typical modal offer)

Responders reject offers below 25% with Responders reject offers below 25% with high probabilityhigh probability

Universal behavior throughout the worldUniversal behavior throughout the world Large degree of variability of offers Large degree of variability of offers

among societies (26 - 58%)among societies (26 - 58%)

Proposer’s optimal strategy: Proposer’s optimal strategy: offer the offer the minimumminimum

Responder’s optimal strategy: Responder’s optimal strategy: accept accept anythinganything

Results:Results:


Extraordinary amount of dataExtraordinary 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)


Paciotti Paciotti et al.et al., , American ScientistAmerican Scientist 9393, 58 (2005), 58 (2005)

Ultimatum game as an interpretingtool

Altruistic rewardingAltruistic rewarding(Fehr, Kirchsteiger & Riedl, 1993)(Fehr, Kirchsteiger & Riedl, 1993)


trustetrusterr

trustetrusteee

MMMM-u-u +2u+2u-v-v+2v+2v


More than 50% of trustees transfer money More than 50% of trustees transfer money back, the more the higher the truster’s back, the more the higher the truster’s transfertransfer

Players often exchangePlayers often exchange M M Universal behaviour throughout the worldUniversal behaviour throughout the worldHomo economicus vs. Homo reciprocansReview in Fehr & Fischbacher, Review in Fehr & Fischbacher, NatureNature 425, 785 (2003) 425, 785 (2003)

Both players gain the maximum by Both players gain the maximum by exchanging exchanging MM The trustee gains more by returning The trustee gains more by returning nothingnothing

Results:Results:So truster’s best strategy is to transfer So truster’s best strategy is to transfer nothingnothing

Strong reciprocityStrong reciprocity

Kin cooperationKin cooperation selfish genes selfish genes ReciprocityReciprocity tit-for-tattit-for-tat strategy strategy

(Axelrod’s experiments)(Axelrod’s experiments) Reputation gainReputation gain altruism as sexual altruism as sexual

ornament ornament (handicap principle)(handicap principle) Strong reciprocity sense of Strong reciprocity sense of fairnessfairness

requires “other-regarding” capabilitiesrequires “other-regarding” capabilities

Explanations for altruistic behaviour:Explanations for altruistic behaviour:


In experiments of public goods, the In experiments of public goods, the presence of a few cheaters quickly presence of a few cheaters quickly deteriorates cooperationdeteriorates cooperation

The introduction of a few The introduction of a few altruistic altruistic punishers punishers forces long term cooperation forces long term cooperation

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

Importance of strong reciprocity:Importance of strong reciprocity:


Standard answer: Standard answer: cultural evolution cultural evolution through group selection through group selection

Evidences in favour:Evidences in favour:– Strong variability between cultures in Strong variability between cultures in

altruistic patternsaltruistic patterns– Increase of altruistic behaviour in Increase of altruistic behaviour in

children as they growchildren as they grow– Simulations of group selection modelsSimulations of group selection models

Evolutionary origins of strong Evolutionary origins of strong reciprocity:reciprocity:

Is group selection unavoidable?

Is evolutionary game theory correct?

Our model: setupOur model: setup

............ NN playersplayers

player player ii

ttii : threshold (minimum share : threshold (minimum share player player ii accepts) accepts)

ffii : fitness (accumulated capital) : fitness (accumulated capital)

MM monetary units ( monetary units (MM=100) =100)

A.S. & J. A. Cuesta, A.S. & J. A. Cuesta, J. Theor. Biol., J. Theor. Biol., in press (2005)in press (2005)

Our model: game stepOur model: game step


proposeproposerr

responderesponderr

tt

pp

ffpp

ttrr

ffrr

ttp p

ttrr

≥≥

+M-+M-ttpp

+t+tpp

ttp p ttrr <<

Our model: every Our model: every ss stepssteps


minimuminimum m

fitnessfitness

maximumaximum fitnessm fitness

ttmimi

nn

ffminmin

ttmaxmax

ffmaxmax

new new playerplayer

t’t’maxmax

ffmaxmax

mutation: mutation: t’t’maxmax==

ttmaxmax

(prob.=1/3(prob.=1/3))

+1 +1 -1 -1

Implemented featuresImplemented features

Degree of altruism inherited from Degree of altruism inherited from progenitorprogenitor (with some error) (with some error)

No learning from experienceNo learning from experience No influence from other playersNo influence from other players (no (no

culture)culture) No groupsNo groups Other-regarding behaviour with Other-regarding behaviour with

optimizationoptimization (players offer the minimum (players offer the minimum they would accept)they would accept)

Fitness Fitness (accumulated capital) (accumulated capital) is inheritedis inherited

Small populationsSmall populationsN =1000, 107 games, s =1, uniform initial condition

Slow selectionSlow selectionN =1000, 108 games, s =104, ti =1 initial condition

Large populationsLarge populationsN =106, 2x107 games, s =1, uniform initial condition

Slow selectionSlow selectionN =105, 1.28x1011 games, s =104, ti =1 initial condition

Why are Why are reciprocators reciprocators selected?selected? Higher threshold implies lower payoffs but Higher threshold implies lower payoffs but

higher acceptance rateshigher acceptance rates for agent’s own for agent’s own offersoffers

The presence of altruistic punishers The presence of altruistic punishers increases increases rejection of selfish agents’ offersrejection of selfish agents’ offers

Mutations are smallMutations are small (darwinian), so (darwinian), so newborn altruistic agents are just a little newborn altruistic agents are just a little more altruisticmore altruistic

A quantitative description is still lackingA quantitative description is still lacking

Two thresholds (s Two thresholds (s small)small)

N =1000, 106 games, s =1, uniform initial condition

acceptaccept

offeroffer

Two thresholds (s Two thresholds (s middle)middle)

N =1000, 108 games, s =1000, ti = ai =1 initial condition

acceptaccept

offeroffer

Two thresholds (s Two thresholds (s large)large)

N =1000, 109 games, s =105, ti = ai =1 initial condition

acceptaccept

offeroffer

DiscussionDiscussion

Altruistic punishment is not necessarily a Altruistic punishment is not necessarily a losing strategy when there are many losing strategy when there are many agentsagents

Thus it may be established by individual Thus it may be established by individual selection aloneselection alone

Results reproduce the observed variabilityResults reproduce the observed variability

One or two levels: not important for the One or two levels: not important for the arising of altruistic punishmentarising of altruistic punishment

Mutation rate might be relevantMutation rate might be relevant

Mutation rate effectMutation rate effectN =10000, 108 games, s =1, ti = 1 initial condition

Related resultsRelated results


Stag-hunt game:

• Two equilibria, S and R

• Round-robin gaming: S or Rselected from x0

(x fraction of R)

• s games between death-birth

S R

C. P. Roca, J. A. Cuesta & A. S., in progress (2005)C. P. Roca, J. A. Cuesta & A. S., in progress (2005)


Random death Death proportional to fitness

DiscussionDiscussion In general, evolutionary game theory In general, evolutionary game theory

studies a limit situation: studies a limit situation: s infinite! s infinite! (every player plays every other one (every player plays every other one before selection)before selection)

Number of games per player Poisson Number of games per player Poisson distributeddistributed

Fluctuations may keep players with Fluctuations may keep players with smaller ‘mean-field’ fitness alivesmaller ‘mean-field’ fitness alive

Further discussionFurther discussion Thresholds observed in neural activity Thresholds observed in neural activity

measurements measurements (Sanfey et al., 2003) (Sanfey et al., 2003) (suggest that this threshold is an (suggest that this threshold is an inheritable feature) inheritable feature)

Correlation with other-regarding Correlation with other-regarding behavior: also in different primates behavior: also in different primates (capuchin monkeys) (capuchin monkeys) (Brosnan & de (Brosnan & de Waal, 2003)Waal, 2003)

Main conclusionsMain conclusions

Altruism Altruism (altruistic punishment, as (altruistic punishment, as implemented in the Ultimatum Game)implemented in the Ultimatum Game) need need not be an evolutionary losing strategy, not be an evolutionary losing strategy, so it so it may have arisen through standard may have arisen through standard individual selectionindividual selection

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

Adam Smith on Adam Smith on altruismaltruismHow 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)

on the evolutionary origins of altruistic behavior: can selection at the individual level be enough?...

Documents

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