In this blog post, we’ll examine the phenomenon of “birds of a feather flock together,” in which altruistic people tend to gather in groups, and explore how this phenomenon influences cooperative behavior and the emergence and evolution of altruistic individuals.
“No! Pass the ball to me!” Hyun-jun, playing soccer during gym class, yells at Heung-min, who lost possession after attempting a risky dribble instead of passing to Hyun-jun—who was standing unmarked with no defenders nearby. As similar situations repeat throughout the game, Hyun-jun silently vows to himself that the next time the teams are chosen, he won’t be on the same team as Heung-min. As seen in the scenario above, in many situations throughout life, individuals tend to work or socialize with altruistic people—those who pass the ball well—while trying to keep their distance from selfish people—those who attempt to dribble past defenders on their own. The “Birds of a Feather Hypothesis” posits that when altruistic people gather together, it creates an environment conducive to the maintenance and evolution of cooperative behavior—that is, passing the ball to one another to create scoring opportunities. In this article, we will explore how altruistic people come together and examine how the Birds of a Feather Hypothesis supports the emergence of altruistic humans.
Before explaining why altruistic people tend to cluster together, I will introduce research findings showing that people with similar traits—particularly similar cultural and behavioral characteristics—tend to cluster together. Jared Diamond, who conducted research in geography and physiology at UCLA, cited a study that examined the degree of similarity between spouses in terms of physical characteristics, personality, political views, and economic status. Diamond served as a professor of geography at UCLA for many years and retired in 2024. The results showed that the correlation coefficients for physical characteristics and personality were 0.2 and 0.4, respectively, whereas the coefficient for political views reached 0.9. Here, a correlation coefficient closer to 1 indicates that similar traits cluster together; closer to -1 indicates that opposing traits cluster together; and closer to 0 means there is no correlation between the two traits. In other words, while tall people do not necessarily marry other tall people, those with progressive political views often marry others with progressive political views. Therefore, since altruism and selfishness can be viewed as cultural and behavioral characteristics—much like political and religious orientations—we can predict that altruistic people tend to associate with other altruistic people; in other words, “birds of a feather flock together.”
To put it simply, when dividing into teams to prepare for a debate, you would likely prefer to work with a friend who collaborates and shares ideas rather than one who only prepares their own remarks, and that friend would also want to team up with someone similar. In particular, the more information one has about another person—such as observations made in daily life or stories heard from others—the stronger the tendency to associate with altruistic individuals becomes.
Furthermore, the longer a group persists, the more its own rules and norms emerge. Those who violate these rules and act selfishly are gradually weeded out of the group through rules or disciplinary measures, leading to a “birds of a feather flock together” phenomenon where only altruistic individuals who do not harm the group remain. As altruistic people are more likely to associate with other altruistic people and less likely to associate with selfish ones, this fosters a positive atmosphere and yields better results. Excluding those who are driven by personal greed and focus solely on dribbling and shooting, when players who are good at passing come together, the soccer team achieves good results through fluid passing plays, and the players can enjoy the game.
As explained above, altruistic people benefit in various ways when they come together, and they gradually separate themselves from selfish individuals who hinder these benefits.
Due to this tendency, people begin to act in ways that show consideration and cooperation toward one another, leading to an increasing number of altruistic individuals.
However, “birds of a feather flock together” does not always lead to positive outcomes. Players might miss the right moment to shoot or make a run because they are too focused on passing to one another. When similar people gather, diversity naturally decreases, and so does creative thinking. Therefore, while “birds of a feather flock together” helps spread altruistic tendencies, it makes it difficult to reap the benefits of diversity that arise from differences—such as expertise or creativity.
When people help one another and move forward together, everyone feels good, and positive outcomes follow. This is why the phenomenon of “birds of a feather flock together” occurs. As a result, through cooperative behavior, the group and its members are maintained and evolve in an altruistic manner. While the “birds of a feather” hypothesis helps us understand the background behind the emergence of altruistic individuals, it inevitably has limitations in producing optimal outcomes because, although the group can become altruistic and move in a positive direction, the disadvantages arising from a lack of diversity and creativity must be considered. Nevertheless, since these limitations can be mitigated by theories such as the “costly signaling hypothesis”—in which individuals demonstrate their superiority through actions, thereby earning the trust of others and prompting altruistic behavior—we can conclude that, for the sake of a better society, altruistic humans have survived the competition with selfish humans and continued to evolve.
