Can you count a friendship? Researchers have tried. One study analyzed 355 survey responses from adults, and a second study surveyed 112 freshmen who had recently moved. In one study, it took roughly 50 hours together to move from acquaintance to casual friend, and it took 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.

Making a friend takes time together, but hours are not the whole story. Some adults reported spending hundreds of hours with coworkers but still called them acquaintances. Time together does not automatically make friends: how people spend their time and what they talk about affects how close they become. One researcher found freshmen who spent one-third of all waking hours in a month with one good friend.

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. Dunbar says social networks are not the same all through. They are highly structured. 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. 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. Baboon friendship studies take years of data: a baboon does not have her first baby until she is 6 or 7.

A chimpanzee study was based on 78,000 hours of observations from more than two decades. As of May 2026, it reported that chimpanzees invested more time in fewer partners, and that bonobos did not show the narrowing of their inner circle with age that chimpanzees showed. Younger male chimpanzees had many one-sided relationships in which grooming was not always returned. As chimpanzees get older, they become more selective about the partners they spend time with, much as aging people do.

A study of nearly 400 children in 4-year-old kindergarten, across 28 classrooms, found they were about 23 percentage points more likely to choose classmates of the same sex to play with. In that classroom study, age differences within the classroom did not significantly influence friendships. The researchers suggested that early classroom play may lay the groundwork for how friendships form later. Girls tend to form smaller, more intimate friendship groups than boys.

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. When people get older, their inner circle becomes smaller because they interact more intensely with fewer other people.

Words to know

• freshmen: first-year students
• structured: organized in layers
• grooming: cleaning each other
• acquaintance: someone you know
• correlated: linked together