Cultural evolution and cultural variation

Peter J. RichersonWith thanks to Robert Boyd

Talk to Radcliffe Exploratory Seminar on Cooperation and Human Systems Design, March 26, 2009

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

Outline of a comprehensive program of studies of cultural evolution

Models of culturalevolution

Individual-levelexperiments

Experimental micro-evolution

Generalevolutionary

theory

Macro-evolutionaryobservations and

experiments

Field observation

Micro-evolutionary

studies

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

Parents

Children

Young Adults

Adults

Other Adults

• Children adopt their parents values

Þ No change

• Young adults preferentially copy entrepreneurial values from other adults

Þ Entrepreneurial values increase

• People with entrepreneurial values are more likely to leave farming

Þ Entrepreneurial values decrease

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

Voting with feet group selection model (with Rob Boyd)

Frequency dependent payoffs to behaviors 1 and 2, e.g. stag huntgame

Payoff biased switching of strategies

Effect of migration on population size

Effect of migration on strategyfrequency in each group

Make migration dependent on themean payoffs in the two groups

In press J. Theoretical Biology

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

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)

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Following Cavalli-Sforza and Feldman, 1973, 1981;A. Bandura, E.M. Rogers

“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)

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 Popperian falibilism– 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

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

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

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

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.

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

Model AIC df weight

INDIV LEARNING 31924.5 2 < 0.001

LINEAR SOCIAL LEARNING 31455.8 3 < 0.001

LIN SL, ADJUST f MIGRANTS 31445.9 4 < 0.001

CONFORMIST SOC LEARN 31429.3 3 0.0012

CON SL, ADJUST f MIGRANTS 31416.8 4 0.63489

LIN and CONF SL 31428.5 4 0.00176

RESIDENT LSL, MIGRANT CSL 31440.9 4 < 0.001

FULLY NESTED MODEL 31417.9 6 0.36215

Results of experiment with migration

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”

Group selection favors cooperation• Groups satisfy Darwin’s postulates

– Groups compete– Groups vary in their ability to survive and reproduce– This variation is heritable

Þ Selection among groups tends to increase the frequency of genes and culture that increase group survival and reproduction

Þ 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

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

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

Ethiopia

U ganda

KenyaZaire

C entra l A frican R epublic

E th iopia

U ganda

KenyaZaire

C entra l A frican R epublic

E th iopia

U ganda

KenyaZaire

C entra l A frican R epublic

The 19th century expansion of the Nuer is an example of cultural group selection

180018401880

Nuer

Dinka

Drawn from data in Kelly 1985

Cultural microevolution in the field

Experiments indicate people punish even when it is not in their self-interest (Fehr and Gächter)

0

4

8

12

1 2 3 4 5 6 1 2 3 4 5 6

Average Investment

Period

Punishment

No punishment

Instincts for cheater detection andaltruistic punishment?

Testing macroevolutionary hypotheses

Au, G nau

Lam alera

M apuche

AchuarO rm aH adza

Sangu

M ongols,Khazaks

Z im babwe

M achiguenga

Tsim aneAche

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

Using the comparative method

UG proposer behavior is variable but nowhere consistent with selfish individual rationalism

L a m a le raA c h é

P i tts b u rg hS h o n aO rm a

A uA c h u a r

S a n g u

G n a u

Tsim a n e

To rg u u dM a p u c h e

H a d z a

Q u ich u aM a c h ig u e n g a

K a z a k h

0 0 .1 0 .2 0 .3 0 .4 0 .5 0 .6 0 .7 0 .8 0 .9 1

U ltim atum G am e O ffe r

Mean OfferMean Offe

L a m a le raA c h é

P i tts b u rg hS h o n aO rm a

A uA c h u a r

S a n g u

G n a u

Tsim a n e

To rg u u dM a p u c h e

H a d z a

Q u ich u aM a c h ig u e n g a

K a z a k h

0 0 .1 0 .2 0 .3 0 .4 0 .5 0 .6 0 .7 0 .8 0 .9 1

U ltim atum G am e O ffe r

Fraction of Offers

An instinct forfair play?

Henrich et al. 2004

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

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

Third party punishment in different societies

Herrmann, Thöni & Gächter, Science 2008

Cross-cultural experiments with the public goods game

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

ENDThanks for your ears!