Scientists long ago understood that "birds of a feather flock together" when it comes to human social networks, with people attracted more easily to those of the same age, physical appearance and ethnic background, as well as other demographic categories.
In the digital era, this tendency extends to social networks too, according to the study, published in the journal Nature Communications.
From an Darwinian perspective, evolutionary psychologists argue, the "like-with-like" principle favour social cohesion, empathy and frictionless collective action.
The relationships we forge with individuals who are clearly different from us -- not of our "tribe" -- tend to be practical, task-oriented and short-lived, research has shown.
One question not answered by the study is whether we naturally gravitate toward people who see the world the same way we do, or whether similarity -- including the way our brains light up -- springs from shared experience.
Most likely, it is a combination of the two.
"We are a social species and live our lives connected to everybody else," said senior author Thalia Wheatley, a professor of psychology and brain science at Dartmouth University in Massachusetts.
"If we want to understand how the human brain works, then we need to understand how brains work in combination -- how minds shape each other."