You Can Only Maintain So Many Close Friendships

The Oxford evolutionary psychologist Robin Dunbar is best known for his namesake “Dunbar’s number,” which he defines as the number of stable relationships people are cognitively able to maintain at once. (The proposed number is 150.) But after spending his decades-long career studying the complexities of friendship, he’s discovered many more numbers that shape our close relationships. For instance, Dunbar’s number turns out to be less like an absolute numerical threshold than a series of concentric circles, each standing for qualitatively different kinds of relationships. He’s also studied the seven factors people use to evaluate whether someone has the potential to become a friend, and the average number of hours it takes for an acquaintance to become a close friend. All of these numbers (and many non-numeric insights about friendship) appear in his new book, Friends: Understanding the Power of Our Most Important Relationships. (The book is out in the United Kingdom, and will be released in the United States in January.)

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