Understanding what other people think and feel, called theory of mind, is linked to both starting and keeping friendships. As children grow, they become more understanding of the differences between themselves and their friends. 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. By about age 12, children understand that trust and support are the bedrock of a quality friendship. By middle school, friends confide in each other.

Making a friend takes time together. In one study, it took roughly 50 hours together to move from acquaintance to casual friend, and about 90 hours together to move from casual friend to friend. A close friend takes more than 200 hours of time together, according to one study of adults. The hours-to-friendship study was published in the Journal of Social and Personal Relationships. Some adults reported spending hundreds of hours with coworkers but still called them acquaintances. People who move to a new place tend to end up with very small social networks until they have embedded themselves. People have circles of friends: about 5 in the closest circle, 15 with the next, then 50, then 150. Time together does not automatically make friends: how people spend their time and what they talk about affects how close they become.

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, and 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. 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. Among primates, the bigger their brains, the larger their social groups, and time spent grooming is correlated with group size.

Human social networks are overwhelmingly homophilous, which means people tend to be friends with people like themselves. In the brain-scan study, researchers first mapped the social network of 279 first-year graduate students, and 42 of them then took part in an fMRI study. The study scanned people's brains with fMRI while they freely watched naturalistic movies. Friends showed more similar brain responses to the same movies than people farther apart in their social network did. The farther apart two people were in a real-world social network, the less similar their neural responses were. The researchers called the large scale of similar responses across friends' brains the most surprising part of the study, and the authors concluded that people are exceptionally similar to their friends.

Words to know

• homophilous: drawn to similar
• fMRI: brain scan
• embedded: settled in
• grooming: cleaning each other