In one experiment, 9- and 14-month-old infants chose graham crackers or green beans. Then they watched a puppet show in which puppets preferred one food or the other. Almost all the infants preferred the character who helped the puppet that was like them. Infants as young as nine months old prefer individuals who are nice to people like them. Babies also respond to familiar faces and show signs of being selective about who they spend time with.

Another test used two adults sitting together and eating. In experiments in the United States, 6-month-old babies watched them. Babies who see two people with similar tastes tend to expect them to be friends. Three- and four-year-olds tend to assume that people are friends if they see them share or help. Experiments indicate that 3-to-5-year-olds associate friendship with acts of loyalty. Many preschoolers describe friends as people who love or care about each other.

Sticker studies add numbers. In one study, preschool children shared more stickers with a friend than with other children. In another, preschoolers gave most stickers to a friend even when the friend had 100 stickers and a child they knew but did not like had only three. Preschoolers will give more to a wealthy friend than to a needy stranger, at least when wealth is measured in stickers. The 5- and 6-year-olds counted friendship for more than the 3- and 4-year-olds did.

Researchers count a friendship as reciprocated when two children each name the other as a friend. Unlike sibling ties, friendships are voluntary and usually involve a high degree of mutual affection and reciprocity. Understanding what other people think and feel, called theory of mind, is linked to both starting and keeping friendships. Friendships are also more common between children who have similar interests.

Making a friend takes time together. In one study, it took roughly 50 hours together to move from acquaintance to casual friend. A close friend takes more than 200 hours of time together, according to one study of adults. Time together does not automatically make friends. How people spend their time and what they talk about affects how close they become.

Dunbar's number is the limit on how many meaningful relationships a person can have at any one time. People have circles of friends: about 5 in the closest circle, 15 with the next, then 50, then 150. Among primates, the bigger their brains, the larger their social groups. Female baboons form close bonds with other females in their troop.

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. As of May 2026, a study of apes reported that chimpanzees invested more time in fewer partners. Younger male chimpanzees had many one-sided relationships in which grooming was not always returned.

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 in middle school, friendships help children learn to express their feelings constructively. By about age 12, children understand that trust and support are the bedrock of a quality friendship. Harvard's long-running study found that close relationships, more than money or fame, keep people happy throughout their lives.

Words to know

• reciprocated: returned by both
• loyalty: sticking by someone
• voluntary: by choice
• constructively: helpfully
• primates: monkeys, apes, people