There are many flavors of friendship. Most U.S. adults say they have pals who fit into specific niches in their lives, like gym friends or work friends. These relationships may come and go as life circumstances change, fading away when someone switches jobs or loses interest in a shared hobby.
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Shared traits, interests, and backgrounds help a lot, says Robin Dunbar, an evolutionary psychologist and author of Friends: Understanding the Power of Our Most Important Relationships. Dunbar’s work suggests there are seven areas of overlap that are particularly crucial in forming a solid friendship: speaking the same language, growing up in the same area, having similar career trajectories, and sharing hobbies, viewpoints, senses of humor, and tastes in music. Every close friend pair may not have every one of these things in common—but the more they share, the stronger their relationship is likely to be, Dunbar says.