This article explains the causes of altruistic behavior by comparing the limitations of various hypotheses, with a focus on the “birds of a feather flock together” hypothesis.
Altruistic behavior refers to acts of helping others even at the cost of personal loss. Common sense tells us that voluntarily engaging in actions that result in personal loss is difficult to understand. So why does this altruistic behavior occur? Scholars have conducted extensive research to explain this behavior and have proposed various hypotheses. Among them, the “birds of a feather” hypothesis is considered quite persuasive; let’s take a closer look at it. First, we will consider what hypotheses existed before the “birds of a feather” hypothesis emerged and what their limitations were. Then, we will examine how the “birds of a feather” hypothesis—which emerged to overcome those limitations—successfully addresses them.
One hypothesis that received significant support in the early days is the kin selection hypothesis. The kin selection hypothesis explains that altruistic behavior arises when individuals in kinship relationships help and protect those with whom they share genes. In fact, family members helping one another has been a common practice throughout history, regardless of time or place. It is not very common for family members to stand by idly when a family member is in trouble. For example, if a younger sibling incurs significant debt due to a business failure, one would help them even while knowing that doing so would result in a financial loss. The kin selection hypothesis effectively explains altruistic behavior that occurs within kinship relationships. However, it has the limitation of failing to account for relationships outside of kinship. Furthermore, in modern society, we live and interact with a wide variety of people who are not related to us by blood. The kin selection hypothesis alone is not sufficient to explain these diverse human relationships.
The repeated reciprocity hypothesis can explain these points. It posits that altruistic behavior arises because interactions between people are repeated. This is a hypothesis that can be applied to more general relationships beyond kinship. Based on human rationality, this hypothesis suggests that if I help someone, they will try to help me in return; if I do not help them, they will act accordingly. In this case, altruistic behavior toward another person functions like a points system for future reciprocity. Consequently, this hypothesis has the advantage of easily explaining altruistic behavior in general relationships, even in the absence of familial bonds such as those found in blood-related families. However, this hypothesis applies only to small groups where sustained interaction occurs. In reality, altruistic behavior can also be observed in large groups where interactions between people do not occur repeatedly.
The “birds of a feather flock together” hypothesis emerged to address this limitation. Whether by choice or circumstance, people tend to gather primarily with those of a similar kind. Here, “similar kind” refers not to physical characteristics but to mental ones. In other words, people with similar mindsets tend to cluster together. In fact, evolutionary biologist Jared Diamond has demonstrated through research that the correlation coefficient between spouses’ ideologies is very high. Consequently, selfish people tend to form groups with other selfish people, while altruistic people tend to form groups with other altruistic people. When altruistic people come together in this way, altruistic behavior can occur even within large-scale groups where sustained interaction is difficult. The “birds of a feather” hypothesis is highly useful in that it provides a mechanism by which altruistic behavior can persist even in large-scale societies.
Since the society to which we belong generally constitutes a large-scale group, the “birds of a feather flock together” hypothesis is highly suitable for explaining altruistic behavior. However, this hypothesis also has clear limitations. Specifically, the broader the group boundaries are defined, the higher the likelihood that selfish and altruistic individuals will be mixed together. In such cases, distinguishing between selfish and altruistic groups becomes meaningless. Furthermore, while the hypothesis dichotomizes human beings into selfish and altruistic individuals, in reality, most people possess a mixture of both traits. Therefore, if the “birds of a feather flock together” hypothesis were to define the size of the group more clearly and apply a multi-category approach rather than a simple dichotomy, it could become an excellent theory that better explains the phenomenon.
In conclusion, altruistic behavior plays a significant role in human society. Various hypotheses have been proposed to explain this behavior, and each provides useful insights within specific contexts. However, to fully explain altruistic behavior, it is necessary to combine the strengths of multiple hypotheses while recognizing the limitations of each. The “birds of a feather flock together” hypothesis is part of this effort and makes an important contribution to understanding altruistic behavior even in large-scale societies. Research on altruistic behavior must continue, as it will enable us to gain a deeper understanding of the complexity and diversity of human society.
