In this blog post, we’ll examine the concept and mechanisms of the group selection hypothesis—proposed to explain altruistic behavior in humans—as well as the role of institutions and the effects and limitations of conformist cultural transmission.
You’ve probably heard the saying, “United we stand!” It means that even in situations that are dangerous when faced alone, the chances of escaping danger increase when people band together. No matter how exceptional someone may be, helping others while accepting a small personal loss can ultimately be of greater benefit to oneself. However, from an evolutionary perspective, if we consider only natural selection at the individual level, it is difficult to explain why someone would help others in situations that offer no direct benefit to their own survival or reproduction. Nevertheless, acts of self-sacrifice to help others are something we encounter several times a day through TV or the internet. Narrowing the scope a bit further, we frequently witness such acts of helping others up close in everyday situations. For example, sharing handwritten notes that one has painstakingly prepared for a friend is an altruistic act that could potentially lower one’s own competitive edge in a competitive situation; therefore, it is difficult to explain from a perspective that considers only direct personal benefit. Thus, there are limitations to explaining human altruistic behavior solely through individual-level choices, and various hypotheses have been proposed to address this; one of them is the “group selection hypothesis.”
In traditional evolutionary theory, while natural selection can occur at the individual level, group selection explains natural selection through differences between groups. The idea is that if a certain behavioral trait increases a group’s chances of survival, the group possessing that trait may gain an advantage in survival and competition over groups that do not. In other words, groups with traits that enhance their chances of survival are more likely to persist longer or spread more widely than other groups due to the influence of group selection. Altruistic behavior—which involves sacrificing oneself to benefit others—is a strategy that incurs a cost at the individual level but can be a superior strategy at the group level. In particular, when examining the development of human civilization, we can identify instances where altruistic cooperation within a group had a significant impact on the group’s survival. For example, the more altruistic individuals there were within a group—and the more willing they were to fight even at the cost of their own lives—the greater the group’s likelihood of winning in intergroup conflicts. Furthermore, even during the hunting-and-gathering era, before human civilization had reached the stage of agricultural societies, altruistic cooperation within a group likely served as a crucial factor determining the success or failure of hunts, thereby having a major impact on the group’s survival. Today’s cultural group selection theory also adopts the idea that intergroup competition and cultural differences can influence human cooperative behavior as a key explanatory framework. However, there remains debate over whether these processes can fully account for all human cooperation and altruism.
Theoretically, however, since altruistic behavior can act in opposite directions at the individual and group levels of selection, it is difficult to determine whether such behavior will survive or disappear as it passes through these two selection processes. If the rate of group selection for altruistic behavior is faster than that of individual selection, altruistic behavior may be more strongly influenced by group selection than by individual selection. However, since changes spreading among individuals generally occur faster than those spreading among groups, the influence of individual selection is likely to be greater. Furthermore, when rewards vary depending on the strategies of individuals within a group, the greater the difference, the greater the pressure from individual selection becomes.
For example, consider Class A, where students who study consistently receive a score of 90, while those who cram the night before the exam receive a score of 50, and Class B, where students who study consistently receive a score of 80, while those who cram before the exam receive a score of 70. The likelihood that students who cram will switch to a strategy of consistent study is greater in Class A than in Class B. In other words, the greater the difference in rewards depending on strategy, the greater the pressure to change behavior at the individual level. However, there is something within a group that mitigates this limitation on speed by ensuring that natural selection operates not only at the individual level but also between groups. That is “institutions.”
If the speed of individual selection can be slowed, the effect of group selection can relatively increase. Institutions can serve to reduce the pressure of individual selection by moderating the differences in rewards associated with different strategies so that they do not become excessively large. For example, a study on income found that the smaller the income gap, the easier it is to reach consensus on the provision of public services. This is because, as income disparities widen, high-income earners bear a greater burden for the same services, leading to greater resistance and, consequently, making it harder to reach consensus on the provision of public services. However, since a system of equal income distribution reduces income disparities, it also reduces the differences in rewards resulting from individual choices. Consequently, such a system can positively influence the choice of altruistic behavioral strategies by alleviating the pressure to choose at the individual level and, relatively speaking, enhancing the effectiveness of group-level choices. In contemporary discussions of cultural group selection, institutions are also treated as a key factor in stabilizing cooperation within a group by altering the reward structure for individual behavior and adjusting the costs and benefits of cooperation and defection.
In human society, there is another phenomenon besides institutions that enhances the effects of group selection. When we model and explain a phenomenon, we typically assume that if characteristic A accounts for 30%, then that characteristic’s influence in society is also 30%. However, according to the theory of “conformist cultural transmission” proposed by Boyd and Richardson, when individuals learn a certain trait, their response is not simply a reflection of that trait’s social frequency but may be biased toward that frequency. In other words, the more prevalent a trait is, the greater the likelihood that individuals will adopt it—even beyond what their proportion in the population would suggest—a process known as conformist transmission. Therefore, regarding altruistic tendencies, as the proportion of people with such tendencies increases, the process by which those tendencies spread socially may be more pronounced than a simple change in proportion would suggest. However, it has not yet been definitively established that this conformist cultural transmission is a key process explaining human cooperative behavior as a whole, and there is debate among researchers regarding the extent to which conformity operates in actual cooperative behavior. For example, if the proportions of altruistic individuals in Groups A and B are 55% and 45%, respectively, the difference between the two groups is only 10%, so the difference between them may not be very noticeable. However, if conformist cultural transmission is strong, altruistic individuals in Group A may exert greater influence than their actual proportion would suggest, allowing altruism to spread more rapidly. Conversely, in Group B, altruistic individuals would have less influence than their actual proportion warrants, while the 55% of self-interested individuals would exert greater influence, allowing self-interest to spread more rapidly throughout society. As a result, the difference in the proportion of altruistic individuals between Group A and Group B could become larger than it was initially, thereby amplifying the effects of group selection, which operates based on these intergroup differences.
The group selection hypothesis extends the level of analysis of natural selection from the individual to the group, providing a framework to explain why altruistic individuals exist. It explains how humans can mitigate the speed disadvantage arising from the group selection process through institutions, and also explains how human cultural learning and transmission, through conformist cultural transmission processes, can amplify intergroup differences and thereby enhance the effects of group selection. However, the group selection hypothesis cannot explain all altruistic behavior. Fundamentally, since the group selection process presupposes differences and competition between groups, it has the limitation that it is difficult to fully explain the emergence and persistence of altruistic behavior in situations where differences between groups are not sufficiently maintained. Furthermore, if differences between groups diminish over time, the effects of selection operating at the group level may also weaken. In fact, while recent research on cultural group selection provides substantial evidence that competition between groups has played a significant role in human society, there is still no clear consensus on whether conformist learning or specific normative psychology serves as the key mechanism for maintaining stable levels of group cooperation.
Nevertheless, the group selection hypothesis has the advantage of explaining why altruistic and cooperative behavior has persisted over a very long period since humans began forming groups. Because it explains how humans created institutions by utilizing their reason and desires and passed down behavioral patterns within groups through cultural transmission, it can be used alongside other evolutionary explanations to understand the emergence of altruistic and cooperative humans. In particular, the group selection hypothesis remains an important subject of research because human cooperation is not formed solely by individual behavior but can emerge through the mutual influence of processes at various levels, such as culture, institutions, and competition among groups. However, it is difficult to view the group selection hypothesis as the sole theory explaining human cooperation and altruism; a more comprehensive understanding of human cooperative behavior requires considering individual-level selection, group-level selection, cultural transmission, and the institutional environment together.
