1. People do not tend to copy cooperative behaviour
A central assumption of CGS models is that individuals copy the behaviour of others and acquire cooperative strategies via social learning. I tested this assumption across 14 populations of the same society, the Pahari Korwa of India (Lamba Reference Lamba2014). A series of public goods games, played with real money and for high stakes, across these populations found little evidence that people used either pay-off biased learning or conformity to copy others' cooperative behaviour. The tendency to copy others' behaviour was highly variable and individuals were less likely to copy more cooperative strategies, that is, behaviour that bore higher costs by depositing more money into a group pot. To my knowledge, this is the only quantitative study conducted in real-world populations that tests whether individuals socially transmit cooperative behaviour, and it suggests that they do not. Hence, the only current quantitative empirical evidence testing whether cooperative behaviour is culturally transmitted does not support this idea.
2. Behavioural variation across human groups is at least partly driven by demographic and ecological processes
Richerson et al. state that “it is an empirical matter whether cultural evolutionary processes can maintain substantial between-group variation” (sect. 2.1, para. 3). They cite studies suggesting that behavioural variation across human groups is driven by corresponding variation in cultural norms of cooperation (Henrich et al. Reference Henrich, Boyd, Bowles, Camerer, Fehr and Gintis2004; Reference Henrich, McElreath, Barr, Ensminger, Barrett, Bolyanatz, Cardenas, Gurven, Gwako, Henrich, Lesorogol, Marlowe, Tracer and Ziker2006; Herrmann et al. Reference Herrmann, Thöni and Gächter2008). The studies (Henrich et al. Reference Henrich, Boyd, Bowles, Camerer, Fehr and Gintis2004; Reference Henrich, McElreath, Barr, Ensminger, Barrett, Bolyanatz, Cardenas, Gurven, Gwako, Henrich, Lesorogol, Marlowe, Tracer and Ziker2006; Reference Henrich, Ensminger, McElreath, Barr, Barrett, Bolyanatz, Cardenas, Gurven, Gwako, Henrich, Lesorogol, Marlowe, Tracer and Ziker2010a; Herrmann et al. Reference Herrmann, Thöni and Gächter2008) administered economic games in up to 16 societies and found that game behaviour varied across them. However, they sampled only one or a few populations per society and therefore did not include population (or village) replicates from each society. Thus, they cannot distinguish whether cultural norms or any of the many other variables that vary between the study populations (e.g., population demographics, habitat variation, climate) cause the observed behavioural differences. Building on their work, we administered public goods games, a naturalistic measure of cooperative behaviour involving the sharing of salt (Lamba & Mace Reference Lamba and Mace2011), and ultimatum games (Lamba & Mace Reference Lamba and Mace2013) in up to 21 populations of the same society, the Pahari Korwa of India. For all three measures, we found significant and substantial behavioural variation across these multiple populations of the same ethno-linguistic group, and we found that some of this variation was explained by demographic factors such as population size, age, and sex. Hence, individuals from different villages did not share a common cultural norm of cooperation despite their common ethnicity, and their behaviour varied in response to the demographic and ecological features of their environment. Our results are bolstered by similar findings in other societies (Falk & Zehnder Reference Falk and Zehnder2013; Gurven Reference Gurven2004; Gurven et al. Reference Gurven, Zanolini and Schniter2008; Nettle et al. Reference Nettle, Colléony and Cockerill2011; Tognetti et al. Reference Tognetti, Berticat, Raymond and Faurie2012). Since demographic characteristics of populations are likely to change during individuals' lifetimes, we may also expect their behaviour to change in response. And if individuals' cooperativeness is likely to change across their lifetimes, then we should not expect culturally heritable variation in the behaviour of human groups across generations.
3. Individuals are more selfish in larger populations and less likely to copy increasingly costly cooperative behaviour
We found that individuals took more salt for themselves and left less for others in larger villages (Lamba & Mace Reference Lamba and Mace2011). Individuals were also less likely to copy more cooperative behaviour (Lamba Reference Lamba2014). While CGS models invoke cultural transmission to explain the evolution of large-scale cooperation, empirical data suggest that cultural transmission is less likely as behaviour gets more cooperative and cooperation is less likely in large populations.
In conclusion, if individuals do not copy cooperative behaviour, the behavioural variation between groups is unstable due to the demographic changes that groups experience over time, and behaviour is more selfish in larger populations, then we must question the existence of stably transmitted cultural norms of cooperation. There is published empirical evidence which Richerson et al. fail to mention, suggesting that the answer to more than one of their questions may be “no.” Therefore, by their own admission, we must also look to hypotheses other than CGS.
Much of my research has focused on seriously testing the cultural group selection (CGS) hypothesis for the evolution of large-scale cooperation in humans as presented by Richerson et al. CGS is an elegant idea that captured my imagination, among those of many others. But a scientist must always remain open to the possibility that even the most elegant of ideas can fail to explain the real world. As Thomas Huxley said, “the great tragedy of Science [is] the slaying of a beautiful hypothesis by an ugly fact” (Huxley Reference Huxley1894, p. 244).
I agree with Richerson et al. that the issue is not whether cooperation based on reciprocity and inclusive fitness exists, but rather whether these explanations can account for large-scale cooperation in humans. I also agree that group selection on cultural variation is theoretically possible, but that the extent of variation, heritability, and competition will determine whether it is an empirically important force or not. Richerson et al. ask, “Are the norms that underpin institutions among the cultural traits so transmitted? Do we observe sufficient variation at the level of groups of considerable size for group selection to be a plausible process?” (target article, Abstract). They state that “If the answer to one of these questions is ‘no,’ then we must look to other hypotheses” (Abstract). My research, as well as that of others, suggests that the answer to both these questions may be “no.”
A series of empirical field studies I conducted in rural India demonstrate that: (1) individuals do not exhibit a strong tendency to culturally transmit cooperative behaviour (Lamba Reference Lamba2014); (2) behavioural variation observed across human groups is at least in part driven by demographic and ecological processes rather than cultural processes (Lamba & Mace Reference Lamba and Mace2011; Reference Lamba and Mace2012; Reference Lamba and Mace2013); and (3) individuals are more selfish in larger populations and less likely to copy increasingly cooperative behaviour (Lamba Reference Lamba2014; Lamba & Mace Reference Lamba and Mace2011). Thus, I have tested the first two of the three axioms on which CGS is based, that is, cultural heritability, variation, and competition.
1. People do not tend to copy cooperative behaviour
A central assumption of CGS models is that individuals copy the behaviour of others and acquire cooperative strategies via social learning. I tested this assumption across 14 populations of the same society, the Pahari Korwa of India (Lamba Reference Lamba2014). A series of public goods games, played with real money and for high stakes, across these populations found little evidence that people used either pay-off biased learning or conformity to copy others' cooperative behaviour. The tendency to copy others' behaviour was highly variable and individuals were less likely to copy more cooperative strategies, that is, behaviour that bore higher costs by depositing more money into a group pot. To my knowledge, this is the only quantitative study conducted in real-world populations that tests whether individuals socially transmit cooperative behaviour, and it suggests that they do not. Hence, the only current quantitative empirical evidence testing whether cooperative behaviour is culturally transmitted does not support this idea.
2. Behavioural variation across human groups is at least partly driven by demographic and ecological processes
Richerson et al. state that “it is an empirical matter whether cultural evolutionary processes can maintain substantial between-group variation” (sect. 2.1, para. 3). They cite studies suggesting that behavioural variation across human groups is driven by corresponding variation in cultural norms of cooperation (Henrich et al. Reference Henrich, Boyd, Bowles, Camerer, Fehr and Gintis2004; Reference Henrich, McElreath, Barr, Ensminger, Barrett, Bolyanatz, Cardenas, Gurven, Gwako, Henrich, Lesorogol, Marlowe, Tracer and Ziker2006; Herrmann et al. Reference Herrmann, Thöni and Gächter2008). The studies (Henrich et al. Reference Henrich, Boyd, Bowles, Camerer, Fehr and Gintis2004; Reference Henrich, McElreath, Barr, Ensminger, Barrett, Bolyanatz, Cardenas, Gurven, Gwako, Henrich, Lesorogol, Marlowe, Tracer and Ziker2006; Reference Henrich, Ensminger, McElreath, Barr, Barrett, Bolyanatz, Cardenas, Gurven, Gwako, Henrich, Lesorogol, Marlowe, Tracer and Ziker2010a; Herrmann et al. Reference Herrmann, Thöni and Gächter2008) administered economic games in up to 16 societies and found that game behaviour varied across them. However, they sampled only one or a few populations per society and therefore did not include population (or village) replicates from each society. Thus, they cannot distinguish whether cultural norms or any of the many other variables that vary between the study populations (e.g., population demographics, habitat variation, climate) cause the observed behavioural differences. Building on their work, we administered public goods games, a naturalistic measure of cooperative behaviour involving the sharing of salt (Lamba & Mace Reference Lamba and Mace2011), and ultimatum games (Lamba & Mace Reference Lamba and Mace2013) in up to 21 populations of the same society, the Pahari Korwa of India. For all three measures, we found significant and substantial behavioural variation across these multiple populations of the same ethno-linguistic group, and we found that some of this variation was explained by demographic factors such as population size, age, and sex. Hence, individuals from different villages did not share a common cultural norm of cooperation despite their common ethnicity, and their behaviour varied in response to the demographic and ecological features of their environment. Our results are bolstered by similar findings in other societies (Falk & Zehnder Reference Falk and Zehnder2013; Gurven Reference Gurven2004; Gurven et al. Reference Gurven, Zanolini and Schniter2008; Nettle et al. Reference Nettle, Colléony and Cockerill2011; Tognetti et al. Reference Tognetti, Berticat, Raymond and Faurie2012). Since demographic characteristics of populations are likely to change during individuals' lifetimes, we may also expect their behaviour to change in response. And if individuals' cooperativeness is likely to change across their lifetimes, then we should not expect culturally heritable variation in the behaviour of human groups across generations.
3. Individuals are more selfish in larger populations and less likely to copy increasingly costly cooperative behaviour
We found that individuals took more salt for themselves and left less for others in larger villages (Lamba & Mace Reference Lamba and Mace2011). Individuals were also less likely to copy more cooperative behaviour (Lamba Reference Lamba2014). While CGS models invoke cultural transmission to explain the evolution of large-scale cooperation, empirical data suggest that cultural transmission is less likely as behaviour gets more cooperative and cooperation is less likely in large populations.
In conclusion, if individuals do not copy cooperative behaviour, the behavioural variation between groups is unstable due to the demographic changes that groups experience over time, and behaviour is more selfish in larger populations, then we must question the existence of stably transmitted cultural norms of cooperation. There is published empirical evidence which Richerson et al. fail to mention, suggesting that the answer to more than one of their questions may be “no.” Therefore, by their own admission, we must also look to hypotheses other than CGS.