Birds of a feather flock together. Can this popular saying be seen in the brain? Empirically speaking, sociology has shown that we associate with people that are like us; something known as (social) homophily. American researchers have studied this phenomenon on a neural level. As incredible as it may seem, they were able to identify a person’s friends by analyzing their reactions in the brain while watching video clips! Let’s take a closer look.

The intuition that we tend to choose friends who are like ourselves has been confirmed through research on homophily. The sociological variables that help forge social ties include social origin, age, education, place of residence, socio-professional environment, an even physical characteristics. Some studies go so far as to suggest that homophily is an ancient organizing principle, and perhaps the most robust rule guiding human sociability. Others indicate that cases of heterophily (the opposite tendency: to associate with people who are dissimilar to ourselves) are much rarer and usually exist only to accomplish an explicit goal (e.g. a professional collaboration).

According to the authors of this study, certain personality traits can also show “social assortativity.” Thus, people may group together according to their world vision, or common values and interests. So how do you test this human tendency to associate with other people who have the same world view? C. Parkinson and colleagues hypothesized that the cognitive processes may be similar among friends, and they developed a protocol to show that homophily can also be neural.

First, the team of researchers characterized the social network (the friendship links) of a cohort of 279 students (89 women, 190 men) from a private US university. They were given an online questionnaire with questions such as: “Consider the people with whom you like to spend your free time. Who are the classmates you have been with most often for informal social activities, such as going out to lunch, dinner, drinks, films, visiting one another’s homes, and so on?” Then, to test whether this social network mapping could be seen on a neural level, the scientists selected 42 students ages 25 to 32 (12 women, 30 men). Participants were asked to watch a series of videos covering subjects like politics, science, sports, humor and music while undergoing a brain scan using fMRI. Each volunteer watched the clips in the same order. The researchers analyzed the neural responses in pairs in order to determine if the students who claimed to be friends had more similar brain activity than those who were more distant from one another. The scientists found that the neural responses were significantly more similar in the first case. The similarities in neural responses are particularly visible “in brain regions involved in attentional allocation, narrative interpretation, and affective responding.” It should be noted that these observations remain significant even when controlling for demographic variables (age, gender, nationality, etc.).

The results of this study indicate the existence of neural homophily: individuals tend to befriend people who see the world in the same way and react similarly emotionally. The authors indicate that it would be possible to predict not only whether two people are friends, but also the social distance that separates them, simply by looking at how their brains react to videos!