A study by scientists at Britain's University of York found that our facial recognition abilities allow us to process the thousands of faces we encounter in busy social environments, on our smartphones and our television screens every day.
"In everyday life, we are used to identifying friends, colleagues, and celebrities, and many other people by their faces," Rob Jenkins, from York's Department of Psychology, told AFP.
"But no one has established how many faces people actually know."
For the study, published in the journal Proceedings of the Royal Society B, Jenkins and his team asked participants to write down as many faces they could remember from their personal lives.
The volunteers were then asked to do the same with people they recognized but did not know personally.
They were also shown thousands of images of famous people — two photos of each to ensure consistency — and asked which ones they recognized.
The team found an enormous range of the number of faces each participant could recall, from roughly 1,000 to 10,000.
"We found that people know around 5,000 faces on average," Jenkins said. "It seems that whatever mental apparatus allows us to differentiate dozens of people also allows us to differentiate thousands of people."
The team said it believes this figure — the first ever baseline of human "facial vocabulary," could aid the development of facial recognition software increasingly used at airports and criminal investigations.
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It may also help scientists better understand cases of mistaken identity.
"Psychological research in humans has revealed important differences between unfamiliar and familiar face recognition," said Jenkins.
"Unfamiliar faces are often misidentified. Familiar faces are identified very reliably, but we don't know exactly how."
While the team said it was focused on how many faces humans actually know, they said it might be possible for some people to continue learning to recognize an unlimited number of faces, given enough practice.
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They pointed out that the brain has an almost limitless capacity to memorize words and languages — the limits on these instead come from study time and motivation.
The range of faces recognized by participants went far beyond what may have been evolutionarily useful: for thousands of years humans would likely only have met a few dozen people throughout their lives.
Jenkins said it was not clear why we developed the ability to distinguish between thousands of faces in the crowd.
"This could be another case of 'overkill' that is sometimes seen in nature," he said.
"The venom of some spiders can kill a horse, even though the spider has no need to eat a horse."