Next time you struggle to put a name to a face, go easy on yourself.
You probably recognise thousands of people.
Participants in a British study recognised 1 000 to 10 000 faces, with the average number being an astonishing 5 000.
The faces included people they knew from their personal lives, as well as famous people.
"Our study focused on the number of faces people actually know - we haven't yet found a limit on how many faces the brain can handle," said Rob Jenkins, a reader in the department of psychology at the University of York in England.
"The ability to distinguish different individuals is clearly important - it allows you to keep track of people's behaviour over time, and to modify your own behavior accordingly," he said in a university news release.
The findings offer a baseline for comparing the "facial vocabulary" of people with facial-recognition software now used to identify people in airports and police investigations.
Jenkins offered several possible explanations for the large range in number of faces people recognised.
Some people may have a natural aptitude for remembering faces, he said. People also differ in how much attention they pay to faces, and how efficiently they process information.
"Alternatively, it could reflect different social environments - some participants may have grown up in more densely populated places with more social input," Jenkins added.
The average age of the study volunteers was 24. Age is a fertile topic for further study, Jenkins said.
"It would be interesting to see whether there is a peak age for the number of faces we know," he said. "Perhaps we accumulate faces throughout our lifetimes, or perhaps we start to forget some after we reach a certain age."
The report was published October 10 in the Proceedings of the Royal Society B.