For the first time, researchers have pinned down the number of faces that people remember; the findings come from a small study of 25 people ages 18 to 61 years old. The answer: 5,000 on average and up to 10,000.
The scientists tested participants' ability to recognize the faces of people the subjects knew personally and the faces of famous people, finding that the participants could identify 5,000 faces on average as being "familiar" to them.
In the study, the participants were asked to make lists of people from their lives whose faces they could picture or whom they would easily recognize if they saw them in person. The scientists also asked the subjects to identify celebrities from thousands of photos.
Participants were not required to attach names to the faces they said they knew, as "naming is clearly separable from visual recognition," the scientists explained.
From the study's findings, it's clear that facial-recognition ability varies widely between individuals; some people could remember as few as 1,000 faces and some as many as 10,000, and some people clearly have a tougher time than others recognizing faces. Anecdotal evidence outside of the study underscores this. For example, the actor Brad Pitt gave an interview in 2013 about having difficulty remembering people that he'd met before, mentioning that he "can't grasp a face."
A condition called prosopagnosia, also known as face blindness, impedes facial recognition and may affect millions of people, Live Science previously reported. (This does not necessarily include Pitt, who has not been diagnosed with the disorder.)
Studies have also shown that the mechanisms that allow people to recognize faces can vary between cultures, and face blindness tends to be higher when people are looking at faces of another race, according to Scientific American.