People have circles of friends: about 5 in the closest circle, 15 with the next, then 50, then 150. Beyond the 150 closest people, there are layers at about 500 (acquaintances) and 1,500 (faces you can put names to). Dunbar's number is the limit on how many meaningful relationships a person can have at any one time, and Dunbar says social networks are not the same all through, but are highly structured. As of April 2016, a study of 2007 mobile phone calls by 35 million people had found that the average layers held 4.1, 11.0, 29.8 and 128.9 people. Extroverts have more friends than introverts, but still have a similar number of layers, and people who move to a new place tend to end up with very small social networks until they have embedded themselves. Among primates, the bigger their brains, the larger their social groups, and time spent grooming is correlated with group size.
Female baboons form close bonds with other females in their troop, and for female baboons, the quality and stability of relationships matter more than the number of them. Female baboons that kept the same grooming partners from one year to the next lived longer. Baby baboons were more likely to survive when their mothers had close friendships with other female baboons. The baboon researchers said they were not sure exactly how close relationships lead baboons to live longer, and one researcher guessed that well-connected friends could cushion the blow of losing a close friend. Baboon friendship studies take years of data: a baboon does not have her first baby until she is 6 or 7. Younger male chimpanzees had many one-sided relationships in which grooming was not always returned, and as of May 2026, researchers report that as chimpanzees get older, they become more selective about the partners they spend time with, much as aging people do.
Understanding what other people think and feel, called theory of mind, is linked to both starting and keeping friendships. As children get older, how they use theory of mind shifts: preschoolers may use it for pretend play or sharing a joke with a friend. Understanding the perspectives of other people becomes increasingly important for settling conflicts between friends. Children who report a greater willingness to help others are more likely to have high-quality friendships, and teenagers tend to develop higher-quality friendships if their parents are supportive. A review describes friendships as central to children's developing social, emotional and cognitive skills. School-age children learn to deal with disagreements with friends, and preschoolers who are friends have more frequent conflicts than non-friends, but when friends clash they are more likely to settle the matter equitably. Three- and four-year-olds tend to assume that people are friends if they see them share or help, and many preschoolers describe friends as people who love or care about each other.
Harvard's long-running study found that close relationships, more than money or fame, keep people happy throughout their lives. In that study, the people most satisfied in their relationships at age 50 were the healthiest at age 80. Those who kept warm relationships lived longer and happier lives. Unlike sibling ties, friendships are voluntary and usually involve a high degree of mutual affection and reciprocity, and by adolescence, friendships depend on honest exchanges of self-disclosure and affection between two people. Older school-age children start to form lasting, supportive friendships, and by about age 12, children understand that trust and support are the bedrock of a quality friendship. In middle school, friendships help children learn to express their feelings constructively. As children grow, they become more understanding of the differences between themselves and their friends, and young children spend most of the time they are with friends in pretend, imaginative play.
As of May 2026, researchers noted that people invest time and energy in predictable, increasingly wide circles of friends, and that whether other primates do the same had long been unclear. In the ape study, chimpanzees invested more time in fewer partners, and apes living in larger groups were more selective toward group members, as people in larger networks are. Bonobos did not show the narrowing of their inner circle with age that chimpanzees showed. A chimpanzee study was based on 78,000 hours of observations from more than two decades. When people get older, their inner circle becomes smaller because they interact more intensely with fewer other people. As of May 2026, the researchers said the similar friendship patterns in apes and people reveal deep evolutionary continuity in how complex societies are organized.
Words to know
- continuity: unbroken thread
- introverts: quiet people
- embedded: settled in
- grooming: cleaning each other
- correlated: linked together
