Friendships start with demographic similarities, but deepen through other traits

People who are alike are more likely to become friends. This is a well-established sociological finding, summed up in the expression birds of a feather flock together. But in what ways do people actually resemble each other when friendships form? Is it mainly visible traits such as gender and nationality, or more subtle characteristics, like behavior and personality — and does this change as people get to know each other better?

These questions were central to a recent study by sociologist Daniel Cowen and colleagues. They followed a group of students throughout their first year of university, a period in which social networks are largely built from scratch. The setting is ideal for this type of research because students initially know little about each other but spend substantial time together, attending lectures, participating in tutorials, and completing assignments collaboratively.

The researchers mapped who became friends with whom and which similarities played a role. Early on, students tended to connect primarily with others who resembled them in directly visible traits, such as gender and nationality. This is unsurprising, the authors write. When you do not know someone well, these are simply the most obvious cues to rely on.

However, as students got to know each other better, their friendship choices were less dependent on demographics. Friendships became less influenced by what is immediately visible and more by shared interests, behaviors, and attitudes. Whether someone enjoys going out, studies diligently, or is a smart student ultimately mattered more than their background or gender.

The study shows that friendship is not a fixed phenomenon but a process that unfolds over time. First impressions shape whom people initially interact with, but do not necessarily determine who remains a friend in the long term. What starts on the surface gradually shifts toward what connects people in how they think and behave.