Cultural evolution and cultural variation

1. Cultural evolution and cultural variation Peter J. Richerson With thanks to Robert Boyd Talk to Radcliffe Exploratory Seminar on Cooperation and Human Systems Design, March 26, 2009

2. Cultural Evolution Is Darwinian: Culture is a population level process • People’s behavior depends on their skills, beliefs norms, etc. • People’s skills, beliefs and norms depend on the skills, beliefs and norms of their “cultural parents” • Skills, beliefs and norm of cultural parents depend on composition of the population • To predict behavior have to understand why population has cultural composition it does • Evolution is what we call the processes that shape the cultural composition of a population

3. Outline of a comprehensive program of studies of cultural evolution

4. The Darwinian straight and narrow • Set up an accounting system for describing the cultural composition of a population • Determine how processes in the day to day lives of individuals cause the cultural composition to change from one time period to the next • Understand long term change by iterating these microscopic processes over many time periods

5. Parents • Young adults preferentially copy entrepreneurial values from other adults • Entrepreneurial values increase • Children adopt their parents values • No change Children Other Adults Young Adults • People with entrepreneurial values are more likely to leave farming • Entrepreneurial values decrease Adults

6. A Shifting Concatenation of Forces • Random • “Mutation” • Drift • Natural selection • Decision-making • Invention and individual learning • Biased acquisition and teaching (many forms) • Collective decision-making • Policy analysis and policy making • Legal decisions

7. Voting with feet group selection model (with Rob Boyd) In press J. Theoretical Biology Frequency dependent payoffs to behaviors 1 and 2, e.g. stag hunt game Payoff biased switching of strategies Effect of migration on population size Effect of migration on strategy frequency in each group Make migration dependent on the mean payoffs in the two groups

9. Experimental studies of cultural evolution Closely controlled studies of the social learning strategies Cultural evolution in laboratory micro-societies With Richard McElreath, William Baum, Mark Lubell, Charles Efferson, Tim Waring, Adrian Bell, Vicken Hillis

10. Theoretical background to individual decisions experiments: Forces of cultural evolution • Random • “Mutation” • Drift • Natural Selection • Decision-making or psychological forces • Guided variation • Biases • Content based • Frequency dependent • Model based (success) Following Cavalli-Sforza and Feldman, 1973, 1981; A. Bandura, E.M. Rogers

11. “Wheat and Potatoes” experiments • Systematic exploration of individual + social learning • Use simple basic computer driven decision-making task • Two choices • Mean payoffs differ • Noisy environment (variance in payoffs for given mean) • Variable environment (shifts in means) • Controlled exposure to social information • None, individual learning only • One other person’s choices (confirmation strategy) • Several other people’s decisions (conformity) • Other people’s success rate (success bias) • Introduce migration (conformity esp. useful)

12. Information theoretic approach to data analysis • Desire models that maximize the amount of information retrieved from our data • Devise multiple plausible models of how individuals might use individual + social information • Never any guarantee that we have entered the best model into the competition! • Select best fitting model(s) • Information theoretic criteria (AIC, BIC) • An elegant extension of Popperianfalibilism • See Efferson and Richerson, Biology and Philosophy 22: 1-33 (2007) for review or Burnham and Anderson’s 2002 textbook Model Selection and Multimodel Inference: A Practical Information-Theoretic Approach

13. Results of previous experiments • People behave in partial accord with basic theory • Individuals use suboptimal amounts of social information • Lighter than expected dependence on conformity • Individual strategies vary substantially McElreath et al. 2005; Efferson et al, 2008 and Efferson et al., 2008, see my web page

14. Payoffs to self-identified conformists (solid circles) and non-conformists as a function of their estimated Ds

15. Experiment to compare frequency dependent versus payoff dependent biases • Add knowledge of the payoffs to others’ choices • 163 participants • Groups of 4 participants • 60 crop choices per participant • 9780 decisions

16. Participants use a strategy akin to (1) Are the choices’ payoffs similar on average? (2) If yes, go with the more common choice. (3) If no, go with the highest payoff. A sort of pseudo R2 for the HCMFD model is 0.40, and for the HCMINDIV model is 0.37.

17. Experiment with migration • Pure horizontal social learning perhaps unusual • No obvious information asymmetries • Two simultaneous groups • Pairs swapped randomly • Migrants to new environment motivated to use more social information from experienced “farmers” • Induce information asymmetries • 107 participants, 12,840 decisions

18. Results of experiment with migration

19. Macro and microevolution in models and in the field • Some macroevolutionary questions: • How did humans come to be so cooperative? • How did humans come to have such a fancy system for social learning? • Why were big brained humans culturally stagnant and rare between 250 kya and 50 kya? • Why didn’t advanced foragers develop agriculture between 50 kya and 10 kya? • The basic micro-evolutionary problem • Estimate the strength of forces bearing on populations in the ”wild”

20. Group selection favors cooperation • Groups satisfy Darwin’s postulates • Groups compete • Groups vary in their ability to survive and reproduce • This variation is heritable • Selection among groups tends to increase the frequency of genes and culture that increase group survival and reproduction • Group selection tends to increase altruism It must not be forgotten that although a high standard of morality gives but a slight or no advantage to each individual man and his children over other men of the same tribe, yet that an increase in the number of well-endowed men and an advancement in the standard of morality will certainly give an immense advantage to one tribe over another. A tribe including many members who, from possessing in a high degree the spirit of patriotism, fidelity, obedience, courage, and sympathy, were always ready to aid one another, and to sacrifice themselves for the common good, would be victorious over most other tribes; and this would be natural selection. Charles Darwin, Descent of Man, 1871

21. Stable variation between groups leads to cultural group selection • Group selection favors group beneficial social norms • Conformist bias and other processes create and protect group level variation • Quite unlike genes in this regard • Imitation of successful favors group beneficial norms • People ape successful foreigners, hence successful foreign groups • Imitation of successful leads to symbolically marked groups (e.g. ethnic) which cut cultural mixing • Selective migration leads to growth of successful groups • Multiple equilibria and equilibrium selection

22. Cooperation evolved via gene-culture coevolution? • Cultural group selection ⇒ primitive cooperative institutions ⇒ selection for genetic dispositions adapted to cooperation ⇒ potential for more advanced institutions, e.g. moralistic punishment • Symbolic boundary marking (e.g. ethnicity) • Repeated rounds of coevolution⇒ tribal social instincts hypothesis • Guarded ingroup altruism • Willingness to follow rules • Leadership by prestige not dominance • Cultural processes play leading not lagging role • Human “nature “ was substantially constructed by cultural evolution! • The prosocial tribal social instincts act as a moral “hidden hand” • Smith and Darwin had this idea

23. Cultural microevolution in the field The 19th century expansion of the Nuer is an example of cultural group selection 1800 1840 1880 Nuer Dinka Drawn from data in Kelly 1985

24. Experiments indicate people punish even when it is not in their self-interest (Fehr and Gächter) Testing macroevolutionary hypotheses Punishment No punishment Average Investment Instincts for cheater detection and altruistic punishment? Period

25. Using the comparative method Henrich et al. 2001 AER: MacArthur Cross-Cultural Experimental Economics Project: The Ultimatum Game in 15 societies

26. Fraction of Offers UG proposer behavior is variable but nowhere consistent with selfish individual rationalism Mean Offer Mean Offe An instinct for fair play? Henrich et al. 2004

27. Henrich et al. 2006 Science “costly punishment across human societies”

28. Henrich et al. 2006 Science “costly punishment across human societies”

29. Third party punishment in different societies

30. Herrmann, Thöni & Gächter, Science 2008 Cross-cultural experiments with the public goods game

31. Conclusion • Nothing about human cooperation makes sense except in the light of cultural evolution! • Human systems design is an exercise in artificial cultural evolution. • See paper with Collins and Genet, Strategic Organization, 2006, also http://www.collinsff.org/web-content/ceb.html • Manuscript with Joe Henrich, Tribal social instincts and the cultural evolution of institutions to solve collective action problems, on my web site soon END Thanks for your ears!