Researchers at Wellesley College and the University of Kansas investigated friendships at that 25,000-student institution and at four smaller colleges in the state. “People would expect in bigger and more diverse places you’d come into contact with a bigger and more diverse set of people,” says lead researcher Angela Bahns, a social psychologist at Wellesley. “But you find the exact opposite.”

The researchers gave pairs of friends separate questionnaires on their lifestyles (how often they drank, exercised, etc.) and opinions (on topics such as abortion) and found that the bigger the school, the more similar friends were to one another. In follow-up research, not yet published, Ms. Bahns and her team found similar results comparing big cities like New York and Chicago to smaller ones like Iowa City and Lawrence, Kan.

How can more people and more diversity lead to less diverse friendships? It’s simple, really: We like people who are like us. Social scientists call it the “similarity-attraction effect,” and it influences everything from whom we date and hire to where we choose to live. The bigger the pond, the more likely we are—consciously or not—to swim around until we find a group of like and like-minded people.