In this blog post, we’ll explore why altruistic behavior evolved and persisted despite the selfishness inherent in human nature, using group selection theory.
In our society, altruistic acts such as donating blood or cleaning up alleys are commonplace. However, viewed through the lens of natural selection, this is nothing short of a mystery. This is because altruistic individuals help others at a personal cost, while selfish individuals can receive help without any effort of their own, making it highly likely that the altruistic will be eliminated in the competition for survival between the two groups. According to natural selection, members of society would adopt selfish strategies that give them an advantage in the competition for survival, so selfishness would eventually become rampant in our society. However, contrary to this expectation, altruistic behavior still abounds in our society. Even though they could gain greater benefits by free-riding, people suppress their selfish impulses and act altruistically. Where does this altruism actually come from?
One of the keys to solving this puzzle is the “group selection hypothesis.” Group selection is the hypothesis that, just as individual traits affect an individual’s survival, group traits also affect the survival of the group. In other words, the object of natural selection is not the “individual” but the “group.” Altruistic behavior, which could be a weakness in individual selection, can actually become a strength for the group in group selection, giving it an advantage in the struggle for survival. For example, consider the situation in the game StarCraft where a Defiler unit sacrifices other Zerglings to restore its energy. If there were many selfish Zerglings, none would be willing to sacrifice themselves, making it difficult to win battles between species. On the other hand, if there were many altruistic Zerglings, they would willingly sacrifice themselves, thereby contributing to the victory of the entire species.
Looking at human history, there are periods when group selection likely played a significant role. In primitive tribal societies, wars between tribes were very frequent, and the mortality rate was also high. Groups with many altruistic individuals would have been more likely to prevail in these inter-tribal conflicts. Even in hunter-gatherer societies preceding agricultural societies, the success of hunting and gathering depended heavily on how altruistically members cooperated. In the harsh environments spanning tens of thousands of years, altruistic cooperative behaviors—such as finding safe shelter and protecting one another from external threats—were essential. Groups with a higher proportion of altruistic individuals would have been more likely to survive in harsh environments.
However, this hypothesis still faces unresolved issues. Under individual selection, altruistic individuals struggle to succeed compared to selfish ones and ultimately face the risk of extinction. Under group selection, however, groups with many altruistic individuals are more successful or have an advantage in the competition for survival than those without. Since individual selection and group selection act in opposite directions, for the group selection theory to be convincing, the pace of group selection must be able to overwhelm that of individual selection.
Humans have slowed the pace of individual selection and amplified the effects of group selection through “institutions”—such as rules, customs, and laws—that influence interactions among members of society. One such institution is the equal distribution of income. The food-sharing customs of ancient hunter-gatherer tribes acted as a mechanism for equal income distribution, reducing the difference between altruistic and selfish individuals and thereby increasing the likelihood of group selection.
Conformist cultural transmission is another factor that has allowed group selection to exert a strong influence. Conformist cultural transmission means that if altruistic individuals make up more than half of a group, the number of people willing to adopt altruistic strategies increases, leading to more altruistic individuals; conversely, if selfish individuals make up more than half, the number of people willing to adopt selfish strategies increases, leading to more selfish individuals. Conformist cultural transmission is significant because even minor differences within a group can lead to major differences between groups.
For example, suppose there are two groups: one with 55% altruistic individuals and another with 45%. Although the difference between the two groups is only 10%, if conformist cultural transmission occurs within each group, the number of altruistic individuals in the first group will gradually increase, while in the second group, altruistic individuals will gradually disappear. In this way, conformist cultural transmission widens the gap between groups, thereby amplifying the effects of group selection.
So far, we have used the theory of group selection to solve the puzzle of how altruistic individuals have been able to survive. Group selection is the hypothesis that natural selection occurs at the group level, and that the characteristics of a group influence its survival. Humans have created an environment where altruistic individuals can survive by maximizing the effects of group selection through institutions and conformist cultural transmission. Although group selection has the limitation of being evolutionarily unstable, it is a compelling hypothesis for explaining how altruistic behavior has evolved in human society.
