Even though homophily (love of the same) is often thought of as a standard feature of friendships, the empirical evidence for attraction based on personality trait similarity is mixed at best. One reason for the inconsistent findings across studies could be variation in the large‐scale social environment in which the studies have been conducted. We investigated whether diversity in the everyday social ecologies of 7‐ to 8‐year‐old children (N = 549) moderates whether friendships are formed on the basis of similar personality traits and similar levels of Cognitive ability. Moderated polynomial regression and response surface analyses showed that classroom size moderated homophily based on Openness to Experience: children similar in Openness were more likely to form friendship ties, but only in larger classrooms. Moreover, we found homophily for Cognitive ability, especially among girls. The results for Openness and Cognitive ability were independent of each other. We discuss the social relevance of trait Openness and the notion that capacity to reciprocate underlies homophily based on Cognitive ability. Copyright © 2017 European Association of Personality Psychology