Transcript Cultural evolution and cultural variation

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
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
General
evolutionary
theory
Microevolutionary
studies
Models of cultural
evolution
Macro-evolutionary
observations and
experiments
Individual-level
experiments
Experimental microevolution
Field observation
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
• Children adopt their
parents values
 No change
Parents
Children
Young Adults
Adults
Other Adults
• Young adults preferentially
copy entrepreneurial values
from other adults
 Entrepreneurial values
increase
• People with
entrepreneurial values are
more likely to leave farming
 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)
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
Experimental studies of cultural
evolution
Closely controlled studies
of the social learning
strategies
Cultural evolution in
laboratory microsocieties
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
p'  p1  q1  q2   q1
p'  p  Bp 1  p
• Content based
• Frequency dependent __p'  __
p  Dp1  p2 p 1
• Model based (success) X '  X  1  1 / n e CovZ X , Z I 
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 nonconformists 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
Results of experiment with migration
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
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
 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
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
Cultural microevolution in the field
The 19th century expansion of the Nuer
is an example of cultural group selection
1800
1880
1840
Ethiopia
Nuer
Central African
Republic
Dinka
Zaire
Kenya
Uganda
Drawn from data in Kelly 1985
Testing macroevolutionary hypotheses
Experiments indicate people punish even when it is not in their self-interest (Fehr and Gächter)
Punishment
12
No punishment
Average
Investment
8
Instincts for cheater
detection and
altruistic punishment?
4
0
1
2
3
4
5
6
Period
1
2
3
4
5
6
Using the comparative method
Henrich et al. 2001 AER: MacArthur Cross-Cultural Experimental Economics Project:
The Ultimatum Game in 15 societies
Mongols,
Khazaks
Lamalera
Achuar
Orma
Hadza
Sangu
Machiguenga
Au, Gnau
Tsimane
Ache
Zimbabwe
Mapuche
UG proposer behavior is variable but nowhere consistent with selfish
individual rationalism
Lamalera
Aché
Pittsburgh
Shona
Orma
Au
Achuar
Sangu
Gnau
Tsimane
Kazakh
Torguud
Mapuche
Hadza
Machiguenga
Quichua
Mean Offe
Offer
Fraction of Offers
An instinct for
fair play?
0
0.1 0.2 0.3 0.4 0.5 0.6
0.7 0.8
Ultimatum Game Offer
0.9
1
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
END
Thanks for your ears!