Thursday, June 10, 2021

New social interaction mechanisms—change in the rules of exchange and collective-action dilemmas—devised by the interacting individuals, allow for self-interested individuals to remain prosocial as societies grow

Cooperation in large-scale human societies—What, if anything, makes it unique, and how did it evolve? Simon T. Powers, Carel P. van Schaik, Laurent Lehmann. Evolutionary Anthropology: Issues, News, and Reviews, June 4 2021,

Abstract: To resolve the major controversy about why prosocial behaviors persist in large-scale human societies, we propose that two questions need to be answered. First, how do social interactions in small-scale and large-scale societies differ? By reviewing the exchange and collective-action dilemmas in both small-scale and large-scale societies, we show they are not different. Second, are individual decision-making mechanisms driven by self-interest? We extract from the literature three types of individual decision-making mechanism, which differ in their social influence and sensitivity to self-interest, to conclude that humans interacting with non-relatives are largely driven by self-interest. We then ask: what was the key mechanism that allowed prosocial behaviors to continue as societies grew? We show the key role played by new social interaction mechanisms—change in the rules of exchange and collective-action dilemmas—devised by the interacting individuals, which allow for self-interested individuals to remain prosocial as societies grow.


As discussed in Section 2, large-scale societies are dependent on prosocial behaviors for their existence. Yet, evolutionary theory shows that, everything else being equal, the selection pressures favoring prosocial behaviors decrease drastically as the number of interacting individuals increases, and hence prosocial behavior is unlikely to be favored in large groups (Powers & Lehmann, 2017). So what was the key mechanism that allowed for prosocial behavior to be sustained in the transition from small-scale to large-scale societies? Did prosocial behavior start to go against the actor's genetic self-interest as the scale of society increased? If so, it would need to be maintained by a decision-making mechanism that is less sensitive to material payoff compared to the decision-making mechanism of evolutionary biology (FMM) described in the previous section. Or alternatively, was prosocial behavior still payoff-sensitive because of changes in the social interaction mechanism?

4.1 Hypothesis 1: A special decision-making mechanism is the key driver of prosocial behavior in large-scale societies

The first hypothesis is the cultural group selection hypothesis (Richerson et al., 2016; Richerson & Boyd, 2005), which posits that the main driver maintaining the expression of prosocial behavior in the transition to large-scale societies is that humans are largely SLM agents with a high degree of prestige and conformity bias. These biases can maintain prosocial behavior within a group, even if the prosocial behavior is not payoff-sensitive and hence not an equilibrium behavior for self-interested agents. Prosocial acts in large-scale societies can therefore be altruistic under this hypothesis. If different groups reach different patterns of behavior, some with a greater frequency of prosocial behaviors, and some with less, then competition between groups can cause the prosocial behavior to spread throughout the population (Boyd et al., 2003; Henrich & Boyd, 2001). However, explicit formal models investigating these processes (Lehmann & Feldman, 2008; Molleman et al., 2013; Peña et al., 2009), and holding everything else constant in comparison to payoff-biased transmission, have so far generally failed to show that conformist-biased transmission favors the spread of prosocial behaviors. Prestige-biased transmission fares better, though (Molleman et al., 2013).

Cultural group selection was first proposed to explain the transition to large-scale social societies in economics (Hayek, 1988) but without much detail as to what form of competition between groups would do the job, a point that has been expressed as follows (Sugden, 1993):

Sometimes it seems to be suggested that less successful groups imitate more successful ones; sometimes, that individuals migrate from less successful to more successful groups; sometimes, that more successful groups reproduce more rapidly; and sometimes, that more successful groups exterminate less successful ones. I think we must assume that Hayek has no particular theory of group selection clearly in mind, but has the hunch that there is some common criterion of “success” or “fitness” that would be favored by any plausible theory.

Much the same variations of group competition have been proposed in the more recent literature (Richerson et al., 2016). It has been argued that cultural group selection will be reinforced if competition between groups involves the physical displacement of less prosocial groups by their more prosocial neighbors, for example, through warfare (Bowles et al., 2003; Boyd et al., 2003; Turchin et al., 2013). This type of group competition could cause payoff-insensitive prosocial behaviors to spread if SLMs use prestige or conformity bias, including altruistic behaviors. It has also been proposed that competition between groups could take the form of individuals either migrating to more successful groups, or imitating individuals in more successful groups (Richerson et al., 2016). This could cause payoff-insensitive prosocial behaviors to spread if SLMs use prestige bias when choosing the group to migrate to or imitate. In both of these cases, the social interaction mechanism does not matter much because of the agent's decision-making mechanism.

On the other hand, both types of group competition could also function with payoff-biased SLMs. However, in this case there would need to be a mechanism of social interaction which ensures that prosocial behaviors give a higher payoff than non-prosocial behaviors within a single group, for example, non-altruistic forms of sanctioning (Ostrom, 1990). And with payoff-biased SLMs, prosocial behaviors could only be cooperative in large groups of genetically unrelated individuals, and not altruistic. This idea has been proposed in some “weaker” versions of the cultural group selection hypothesis, which argue that prosocial behaviors in social dilemmas are actually cooperative equilibria within a single group (Richerson et al., 2016). However, this is typically an assumption of cultural group selection models (Bowles et al., 2003; Boyd & Richerson, 1990), rather than the models demonstrating the evolution of a social interaction mechanism that makes prosocial behaviors an equilibrium for self-interested individuals in a single group. In summary, there is no single theory of cultural group selection and the different variants make different assumptions on the payoff-sensitivity of individual behavior.

4.2 Hypothesis 2: A refinement of social interaction mechanisms is the key driver of prosocial behavior in large-scale societies

The second hypothesis is the institutional path hypothesis (Powers et al., 2016; Powers & Lehmann, 2017), which posits that the driver maintaining the expression of prosocial behavior in the transition to large-scale societies is a refinement of social interaction mechanisms (see Glossary, Table 1); namely, people changed the rules of their economic games. Individuals are thus assumed to have formal and/or informal political interactions that affect their economic interactions. The hypothesis is that as groups grew in size, individuals have refined and created new institutional rules supporting exchange and/or have changed systems of monitoring and sanctioning to handle larger numbers of individuals in collective-action problems. Institutional rules may also have reduced the effective number of individuals that interact through the creation of nested group structures (Ostrom, 1990). These new mechanisms of social interaction (not necessarily created by “deliberate design,” see more on this in the next section) would lead to prosocial behaviors increasing material payoff to the actor, and hence can be generally favored even by self-interested individuals. Prosociality among non-relatives in large-scale societies is thus always cooperative, rather than altruistic, and so individual behavior is always payoff-sensitive under this hypothesis.

Due to this payoff-sensitivity, the institutional-path hypothesis is compatible with RSM agents, behavioral ecology-biased FMM agents, and with SLM agents that use payoff-biased social learning when choosing prosocial behavior. It is also compatible with the PAM subtype of FMM to the extent that the institutional rules recreate the conditions where cooperative prosocial behaviors were payoff-sensitive in small-scale societies, for example, effective sharing of reputational information. Moreover, PAMs would be expected to create institutional rules similar to those found in small-scale societies in circumstances that are ecologically similar (Boyer & Petersen, 2012; Petersen et al., 2013), for example, to create rules of uniform sharing in periods of high resource variance (Cosmides & Tooby, 1992).

The form of the institutional rules a group ends up with will be influenced by proximate factors such as asymmetries in power, influence, and information (Singh et al., 2017), which determine the outcome of political interactions. Furthermore, only a subset of the individuals affected by the institutional rules may take part in the political interactions, and the interests of those taking part may not be representative of the interests of the group as a whole. Consequently, conflicts of interests between segments of the group may result in institutional rules not being optimal for all group members, as exemplified by the rise of highly despotic states such as Ancient Egypt, where despotic leaders biased institutional rules in favor of themselves. As such, the institutional path hypothesis is compatible with the widespread existence of inefficient institutions (North, 1990). On the other hand, when the interests of group members are aligned, or bargaining strengths are equal, then efficient institutions that increase average material-payoff are more likely to arise, a point that has been repeatedly stressed in the (political) economics literature (Greif, 2006; North, 1990; Ostrom, 1990).

The ability to create and enforce rules by self-interested individuals, especially over food sharing and property rights, would have been necessary to support the hunter-gatherer lifestyle (Hill, 2009). If hunter-gatherers did not have political interactions, then the institutional path hypothesis cannot explain the origin of large-scale societies. But there is evidence that hunter-gatherers do indeed have political interactions that affect their economic interactions, even though they lack the bureaucratic elements of large-scale societies. For example, when the extant Ache hunter-gatherer society transitioned from foraging to horticulture, they advocated and voted in local meetings to transfer fields from public to private ownership (Kaplan et al., 2005).

4.3 A combination of a special decision-making mechanism and a change in social interaction mechanisms?

Since both the cultural group selection and the institutional-path hypotheses assume a quantitative scaling up of the same kinds of exchange and collective-action problems, the above analysis shows that a key question in determining the driving factors in the transition to large-scale societies is what decision-making mechanism determines the expression of prosocial behavior. If individuals are sensitive to material payoff when choosing prosocial behavior, then there must have been a change in their social interaction mechanisms, as proposed by the institutional-path hypothesis. Without such a change, individuals should stop acting prosocially as they took part in exchanges and collective actions with more individuals, because when everything else is constant the pressures favoring prosocial behaviors decrease rapidly as the number of interacting individuals increases. Conversely, if there was no change in the social interaction mechanisms then individuals must be less sensitive to payoff. If so, a special decision-making mechanism must operate, whereby some form of cultural group selection does the work in explaining why prosocial behaviors, be they cooperative or altruistic, are stable in large-scale societies.

Because humans undoubtedly experiment with many behaviors by trial-and-error and do considerably rely on social learning (Legare, 2017), the rules constraining behavior in economic interactions to which a society converge must to some extent at least partly be the outcome of some “spontaneous order” (Hayek, 1988; Sugden, 1993; Vanberg, 1994) and not the outcome of fully deliberate design. A case in point is the advent of the usage of money, which is a typical rule-based change in economic organization that is both in the interest of individuals using it and that is likely to have spread gradually by payoff-biased social learning (Vanberg, 1994). As such, some ingredients of the cultural group selection hypothesis may be complementary with the institutional-path hypothesis, with competition between groups spreading different “spontaneous orders.” However, this depends critically on the exact version of “cultural group selection” that has been operating. If the version of cultural group selection involves altruistic behaviors or altruistic punishment, then it is not complementary as it assumes individuals that are not self-interested (André, 2011; Pinker, 2015).

Competition between groups resulting from warfare, differential migration, or environmentally induced extinctions acts as an equilibrium selection device (Binmore, 2005a; Boyd & Richerson, 1990; Harsanyi & Selton, 1988), favoring equilibria that lead to a higher average payoff for group members. Cultural group selection advocates traditionally stressed that high-payoff equilibria resulted from prestige and conformity biased SLMs causing behaviors to spread within groups even if they were not payoff-sensitive in the underlying social interaction mechanism, rather than being equilibria because they were payoff-sensitive (Henrich et al., 2015; Richerson & Boyd, 2005). But prosocial equilibria can also exist within groups under payoff-biased social learning, or under RSM agents that rationally choose their actions, if the right mechanisms of social interaction are in place. In this case, between-group competition can again act as an equilibrium selection device, spreading by cultural transmission mechanisms of interaction that lead to cooperation, without individuals acting against their self-interest (Binmore, 2005a; Boyd & Richerson, 1990; Harsanyi & Selton, 1988). This can act alongside the creation of mechanisms of interaction by bargaining and negotiation, helping to fill in where individuals are less than fully rational, that is, boundedly-rational RSMs or PAMs. On the other side, an explicit consideration of political interactions for changing institutional rules can complement cultural group selection models, which typically leave unspecified how a group arrives at a particular equilibrium in the first place. Much formal work remains to be done to ascertain the conditions under which such equilibrium selection processes at the level of rules of the game (instead of economic behavior under given rules) may work. There are essentially no models of this to date.

Despite ingredients of the cultural group selection and institutional path hypotheses not being necessarily mutually exclusive, there is a crucial need to understand whether the main driver of the evolution of prosocial behavior in large-scale societies is a special decision-making mechanism that can cause agents to perform prosocial behaviors that are not payoff-sensitive, or the creation of new mechanisms of social interaction that maintain the expression of payoff-sensitive behavior as group size increases. Without clarification, the perennial question of the extent to which prosocial behaviors in large-scale societies are compatible with (genetically) self-interested individuals will remain. Fully elucidating the evolved decision-making mechanism that humans use is extremely challenging. Determining, however, whether observed prosocial behaviors are payoff-sensitive is less challenging. For example, we can more easily determine whether systems of monitoring and sanctioning involve altruistic behaviors, or whether they directly benefit the individuals doing the monitoring and sanctioning by increasing their material payoff (Guala, 2012; Ostrom, 1990). If it is the former, then this suggests that a special decision-making mechanism was key to their spread and maintenance. If it is the latter, then the creation of new mechanisms of social interaction is likely to have been the key driver. Empirical work should thus pay more attention to the payoff sensitivity of monitoring and sanctioning behaviors.

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