How many friends can one person really keep? A British scientist, Robin Dunbar, said about 150. He studied monkeys and apes. Animals with a bigger part of the brain for thinking lived in bigger groups. He used this idea to guess the number for humans.

But in 2021, another team checked his math with new data. Their answers were very different, from a few people to over 500. So the number 150 is still not certain.

Dunbar's number says that people can keep only about 150 stable relationships. In 1992, British anthropologist Robin Dunbar compared many types of primates and found that species with a larger neocortex, the outer part of the brain, tended to live in larger groups. When he applied this pattern to humans, he predicted a group size of about 150. The idea became popular because it seemed to match real groups, such as old villages and army units.

However, in 2021 a team at Stockholm University recalculated the number with newer data and methods. Their estimates ranged from just a few people to more than 500, so they concluded that the number 150 cannot be clearly supported. Dunbar and his colleagues disagree with this criticism.

Dunbar's number, the idea that humans can maintain only about 150 stable relationships, originated in a 1992 study by British anthropologist Robin Dunbar. Comparing primate species, he observed that those with a larger neocortex relative to the rest of the brain tended to live in larger groups.

Extrapolating this relationship to humans yielded a predicted group size of roughly 150, and Dunbar later proposed that human networks are organized in layers of about 5, 15, 50 and 150 people. The figure gained wide appeal because it appeared to match hunter-gatherer bands, traditional villages and military units. In 2021, however, Lindenfors and colleagues at Stockholm University reanalyzed the question using updated data and methods.

Depending on the method, their average estimates ranged from the teens to about 100, and the confidence intervals stretched from a few individuals to more than 500. They concluded that no specific number can be reliably derived from primate data. Critics also note that humans do not process relationships with the neocortex alone and can store social information in tools such as writing and phones. Dunbar's team points to evidence such as Christmas card networks averaging around 150, so the debate continues.