People have circles of friends: about 5 in the closest circle, 15 with the next, then 50, then 150. One study found a sign of friendship in the brain. Friends showed more similar brain responses to the same movies than people farther apart in their social network did. A social network is the web of people someone knows.

Time together does not automatically make friends. How people spend their time and what they talk about changes how close they become. Still, making a friend takes time together. In one study, it took roughly 50 hours together to move from acquaintance to casual friend. An acquaintance is someone you know a little. In one study of adults, a close friend took more than 200 hours of time together.

A baby can react when another child is upset. At first, very young kids play near other kids without interacting much. As they learn language, their play becomes more interactive. Some kids who are not yet in school have one particular friend, and groups of friends can start to form. They often prefer to play with children of the same gender. Friendships are more common between children who have the same interests.

School-age children learn to deal with disagreements with friends. Older school-age children start to form lasting, supportive friendships. By middle school, friends confide in each other, which means they share private things with each other.

Friendship is not only a human thing. Female baboons form close bonds with other females in their troop. A troop is a group of baboons that lives together. Baby baboons were more likely to survive when their mothers had close friendships with other female baboons. As of May 2026, a study reported that as chimpanzees get older, they become more selective (choosy) about the partners they spend time with, much as aging people do.

Words to know

• acquaintance: someone you know
• confide: share private thoughts
• network: web of people
• selective: choosy