In this blog post, we’ll take a closer look at the pros and cons of the “birds of a feather” phenomenon in terms of individual choices and social outcomes.

How to Spot “Birds of a Feather” in Everyday Life

Think back to the plots of the superhero movies you loved as a child. Typically, the hero and the villains are pitted against each other; the villains gather like-minded individuals to join forces, while the hero defeats them with the help of allies who share similar goals. You can see a similar pattern among your friends. While people may resemble each other in appearance or personality, most tend to form close bonds with those who share their hobbies and interests. People generally form relationships based on cultural and behavioral similarities, and this tendency is called “assortative interaction.”
The theory of assortative interaction predicts that altruistic people will associate with other altruistic people, and selfish people will associate with other selfish people. In this article, I will examine in detail how this phenomenon benefits individuals and groups, as well as what limitations it creates.

Mathematical Explanations in Trading and Mating

Given human nature, we can assume that some degree of assortative interaction exists in any group. When pairing people in pairs for trading or games, let s be the probability that two people with similar tendencies will be paired. In the remaining (1-s) probability, pairs are formed randomly. If the proportion of altruistic individuals in this group is p, then the proportion of selfish individuals is (1-p).
Under these conditions, the probability that an altruistic person will encounter another altruistic person can be calculated by combining the following two cases. First, in the s probability range, individuals of the same disposition are always paired; in the (1-s) range—where s does not apply—pairs are formed randomly, so there is a probability of p that an altruistic person will again encounter another altruistic person. Therefore, the overall probability is s + (1-s)·p. This value is always greater than p, which is the probability when there is no “birds of a feather” effect at all.
Calculating in the same way, when “birds of a feather flock together” exists, the probability of altruistic people meeting each other and of selfish people meeting each other both increase, while the probability of people with different tendencies meeting decreases. A higher probability of altruistic individuals meeting means “win–win” outcomes occur more frequently, while a higher probability of selfish individuals meeting implies that situations where they harm each other may arise more often. Furthermore, when an altruistic person meets a selfish person, the altruistic party is more likely to suffer a loss.

The Trap of Homogeneity: Diversity and Innovation

“Birds of a feather flock together” creates a favorable environment for altruistic tendencies to thrive, but groups composed solely of people with the same tendencies may experience slow progress. For example, imagine a class meeting. If all members share similar backgrounds and opinions, the ideas expressed during the meeting are likely to be very similar, making it difficult to expect creative and diverse solutions.
Conversely, an environment where people with different backgrounds and perspectives are appropriately mixed can generate more ideas and unexpected synergies. Therefore, from the perspective of maximizing the happiness and utility of society as a whole, there are limits to the extent of “birds of a feather flocking together,” and finding a balance that maintains an appropriate level of diversity is crucial.

Application and Limitations in Korean Society

Korea has traditionally placed great importance on ties such as kinship, regional connections, and school ties. In this cultural context, understanding the “birds of a feather flock together” phenomenon is essential for social observation and analysis. It is a clear advantage that forming relationships based on shared similarities allows for rapid trust-building and cooperation.
However, if this tendency becomes overly prevalent throughout society, serious problems arise. The homogenization of members’ opinions can hinder social development, and if hiring or promotions are based solely on similarity, it can lead to structural injustice where capable individuals are excluded. In such cases, the simple “like attracts like” effect risks evolving into a “social evil.”
Ultimately, “like attracts like” is a double-edged sword with both advantages and disadvantages. From the perspectives of social development and utilitarianism, a balanced approach is needed: one that leverages the benefits of people grouping with those who are similar while maintaining an appropriate balance that does not compromise diversity and fairness.