In this blog post, I’ll examine how altruistic behavior has been maintained throughout the course of evolution, focusing on the group selection hypothesis, as well as the conditions under which it holds and its limitations.
Why hasn’t altruistic behavior disappeared?
When a selfish person encounters an altruistic person, the selfish person gains a greater advantage in their relationship because they receive help from the altruistic person without offering any in return. Therefore, from an evolutionary perspective, altruistic behavior is considered an inferior survival strategy compared to selfish behavior, and altruistic individuals should have lost the competition for survival and been eliminated during human evolution. However, contrary to this prediction, altruistic behavior can be easily observed in human society and various animal groups today. This discrepancy arises because traditional evolutionary explanations are based on an overly simplified model of competition between selfish and altruistic actors. While the traditional explanation considers only which strategy—selfish or altruistic—yields greater benefits, various hypotheses proposed subsequently seek to explain the continued existence of altruistic behavior today by applying more complex competitive structures. Among these, the group selection hypothesis explains the existence of altruistic behavior by considering not only competition among individuals but also competition among groups. In this paper, we will examine the logic of the group selection hypothesis and the conditions necessary for its validity, and finally, we will explore its limitations.
How does the group selection hypothesis explain altruism?
The group selection hypothesis explains altruism by adding competition between groups to the model of competition between individuals. In intergroup competition, a group with many altruistic actors who sacrifice for the group has a higher probability of survival than a group with many selfish actors who prioritize only their own interests. Consequently, groups with many self-interested individuals are eliminated, while groups with many altruistic individuals thrive.
To understand this, let’s consider a scenario in which two groups are at war. Assume there is no significant difference in the two groups’ technological levels or military capabilities, and that the number of soldiers determines the outcome of the war. Altruistic individuals in each group will actively participate in the war to protect themselves and their community, whereas individuals with selfish tendencies are unlikely to be willing to sacrifice themselves for the group. Therefore, if a group with many altruistic individuals goes to war against a group with many selfish individuals, the group with more altruistic individuals will prevail, and the group with many selfish individuals will be eliminated. As this competition between groups repeats, groups with many self-interested actors will gradually disappear.
This outcome also acts as a pressure that induces individuals within each group to behave altruistically. In the earlier example of war, the reason self-interested actors do not participate in the war is that they judge such a choice to be more beneficial to themselves. However, if this behavior ultimately reduces their chances of survival, they have no choice but to act for the group’s sake—even if only to ensure their own survival. In other words, if competition between groups continues and groups with many self-interested individuals are repeatedly weeded out, self-interested behavior can no longer be an absolutely advantageous survival strategy. At this point, behavior that benefits others simultaneously becomes a strategy that benefits oneself. The group selection hypothesis explains that, as this competition between groups repeats, selfishness gradually decreases, and as a result, altruism has been able to survive to this day.
What are the conditions necessary for the group selection hypothesis to hold?
Since competition not only occurs between groups but also between individuals, for the group selection hypothesis to hold, the rate at which selfishness decreases due to intergroup competition must be faster than the rate at which altruism decreases due to competition between individuals. For this to happen, two conditions are necessary.
The first condition is that intergroup competition must occur frequently enough. Today, conflicts that significantly threaten the survival of a group—such as war—do not occur as frequently as conflicts between individuals. However, throughout human history, armed conflicts between groups have persisted. Anthropological and archaeological research shows that intergroup violence occurred repeatedly in various regions during prehistoric times; however, the mortality rate from war at that time varied greatly depending on the region and era, making it difficult to generalize with a single figure. Thus, it can be argued that this long-term intergroup competition helped create an environment in human society where selfishness became a disadvantageous strategy.
The second condition is that the decline in altruism caused by intragrupp competition must occur relatively slowly. While it is true that intragrupp competition occurs more frequently than intergroup competition, humans have lived in groups and have suppressed excessive selfish behavior through various cultural and institutional mechanisms. Furthermore, many communities in early human societies maintained relatively egalitarian social structures; in such environments, the tendency to imitate selfish behavior for personal success was likely relatively weak. Thus, an environment was formed in human society where the decline in selfishness could occur more rapidly than the decline in altruism, and under these conditions, the group selection hypothesis could operate.
What are the limitations of the group selection hypothesis?
The group selection hypothesis explains that altruistic behavior persists even today because, in situations of intergroup competition, it is a strategy that benefits not only others but also oneself. However, like other hypotheses, the group selection hypothesis also has its limitations.
As intergroup competition repeats, the proportion of altruistic individuals within the surviving groups is likely to gradually converge. Once the proportion of altruistic individuals in each group converges to a certain level, the effect of intergroup competition in reducing selfishness will gradually weaken, and the influence of intragroup competition may become relatively stronger. If this occurs, there is a possibility—similar to models where only intragroup competition exists—that altruism will decline and selfishness will spread.
However, research continues to address these limitations through various theories, such as the “birds of a feather flock together” hypothesis, spatial structure effects, and conformist cultural transmission. Furthermore, despite these limitations, the group-selection hypothesis holds significant importance in that, unlike existing simplistic evolutionary explanations, it suggests that behaviors aimed at helping others or the group can ultimately benefit the individual as well.
