As we move throughout the world, we see a tremendous number of faces—faces of people we know, faces of people we don’t know, faces of celebrities in movies or faces of people in the news. Amidst this vast swirl of encounters, humans are actually quite good at knowing the identities of specific individuals. A new study has found that the average person is capable of recognizing some 5,000 faces, reports Frankie Schembri for Science.

There has been plenty of research on facial recognition in humans, but no previous studies tried to pin down just how many people an individual can recognize. So researchers in the U.K., led by Rob Jenkins at the University of York, set out to come up with a number.

As the team notes in Proceedings of the Royal Society B, the focus of the recent study was not on finding out how many faces a person could feasibly know—that is an issue of memory capacity. Instead, the researchers hoped to determine how many faces the average person actually does know. So they assembled a group of undergraduate and post-graduate students at the University of Glasgow and the University of Aberdeen and asked them to complete two tests.

During the first test, participants were given an hour to recall all of the people that they know personally: family, family friends, personal friends, teachers, schoolmates, professionals like doctors and dentists, and so on. The participants didn’t have to necessarily remember the name of each person they listed; they were permitted to use descriptors like “school janitor,” as long as they could form a clear mental image of the person’s face and feel confident that they would recognize the person if they saw him or her.

Over the course of the hour, the rate at which participants were able to remember faces declined. “To spare participants fatigue,” they weren’t asked to keep going beyond that initial hour, the study authors write. But the researchers were able to extrapolate the data to determine how many faces participants would be able to recall before they ran out of people to list.

For the next phase of the study, participants were shown photographs of 3,441 celebrities and asked to indicate if they recognized them. To make sure they really recognized the individuals in the images, the participants were shown two different pictures of each famous person.

When they combined data from both parts of the study, the researchers found that participants were able to remember between 1,000 and 10,000 faces, with an average of 5,000.

The difference between the top and bottom range “could be explained by some people having a natural aptitude for remembering faces,” Jenkins notes in a statement. "Alternatively, it could reflect different social environments-some participants may have grown up in more densely populated places with more social input."

Speaking to Ian Sample of the Guardian, study author Mike Bruton, a psychologist at the University of York, says the team was “quite surprised by how high the top end was.” The ancient humans from whom we evolved had no need for recalling so many faces, since they typically lived in small groups of less than 100 people.

“Given the social lives of our ancestors, the ability to recognise thousands of individuals might seem like overkill,” Rob Jenkins tells the Guardian’s Sample. “But there are plenty of examples of overkill in nature. The venom of some spiders can kill a horse, even though the spider presumably has no ambitions to eat the horse.”

According to the study authors, having a better understanding of humans’ ability to recall faces can inform the development of facial recognition software, which cannot yet outmatch humans when it comes to recognizing familiar faces. Moving forward, the researchers hope to study how our ability to recognize faces might change as we get older; participants in the recent study ranged in age from 18 to 61, but the mean age was 24.

"It would be interesting to see whether there is a peak age for the number of faces we know," Jenkins says in the statement. "Perhaps we accumulate faces throughout our lifetimes, or perhaps we start to forget some after we reach a certain age."