Hall's first study analyzed 355 survey responses from adults, and his second study surveyed 112 freshmen who had recently moved to Lawrence. 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. Some adults reported spending hundreds of hours with coworkers but still called them acquaintances. The hours-to-friendship study was published in the Journal of Social and Personal Relationships, and one researcher found freshmen who spent one-third of all waking hours in a month with one good friend.

The brain-scan researchers asked whether friends are alike only on the outside or also in how they perceive, interpret and respond to the world. They 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, and 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.

Female baboons form close bonds with other females in their troop, and baboon friendship studies take years of data: a baboon does not have her first baby until she is 6 or 7. 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, and one researcher guessed that well-connected friends could cushion the blow of losing a close friend.

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. A chimpanzee study was based on 78,000 hours of observations from more than two decades. 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. Younger male chimpanzees had many one-sided relationships in which grooming was not always returned.

Researchers count a friendship as reciprocated when two children each name the other as a friend. Naming friends shows researchers who is friends with whom, but it does not tell them how good the interaction between friends is. Popularity and friendship are linked, and the two are not completely separate systems. In one study, a child's popularity in the peer group was not associated with how the child first formed friendships. Robert Selman proposed a timeline of how ideas about friendship change with age, but its stages were based on self-reports.

Words to know

• reciprocated: returned by both
• popularity: being well liked
• naturalistic: like real life
• acquaintance: someone you know
• bonobos: ape species