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Throughout our lives, we commit faces to memory. We can recall classmates from elementary school, neighbors who share our morning commute, and the actors in our favorite television shows. But how many of those faces are stashed in a permanent mental repository?
Researchers at the University of York sought to pin down that number—estimating that people know an average of 5,000 faces. And that figure simply represents the number of faces we might know, not the number we are capable of knowing.
“We may not have an upper limit for face learning,” says Rob Jenkins, the lead author of the study, which was published in the journal Proceedings of the Royal Society B. The thousands of faces we commit to memory may only be the tip of the iceberg for this mental capacity.
Jenkins and his colleagues asked 25 participants, between 18 and 61 years old, to list people whose faces they would clearly recognize. The researchers helped by providing prompts to consider specific social niches, such as colleagues, friends of family, and retail staff. Participants also listed every famous figure they would recognize.
Next, the investigators showed participants a slideshow of famous faces and calculated a ratio based on the number of faces they recognized in the lineup and the number of famous faces they had initially said they would recognize. (This was not done with non-famous faces, since it wasn’t feasible to collect photos of them.) Applying that ratio to participants' self-generated lists of both famous and non-famous faces allowed the researchers to arrive at a final estimate.
The team concluded that on average, participants’ minds held an inventory of roughly 5,000 faces. The capacity differed greatly between participants, ranging from 1,000 to 10,000. That variation may be due in part to the environment in which someone was raised—a rural or urban area, for example—as well as their level of media exposure, Jenkins speculates.
As humans transitioned from living in small, tight-knit communities to a large, interconnected world, the ability for facial learning appears to be up to the task. “It seems that if you’re building cognitive equipment that allows you to differentiate between a couple of hundred individuals, in doing that you’re building apparatus that’s also good for several thousand,” Jenkins says. “Maybe you can’t achieve the former aim without incidentally gaining the additional capacity we use now.”
The collection of faces we know is impressive given that learning a new face requires observing and internalizing the same visage with various expressions, at different ages, and in assorted contexts. A face can look different when its owner puts on makeup, gets a haircut, ages five years, or appears in a darkened restaurant rather than a brightly lit room. “The key to learning each face is learning the person’s variability,” Jenkins says. “You have to be exposed to the way the face changes.” (Importantly, people generally tend to be poor at remembering faces they encountered only briefly, he says. That deficit becomes exceedingly important in legal or forensic situations, such as during eyewitness testimony.)
Identifying how many faces people recognize—and how those faces become familiar—is relevant for understanding deficits of face perception, says Wilma Bainbridge, a post-doctoral researcher at the National Institute of Mental Health who studies the perception and memorability of images. For example, she suggests, face vocabulary could potentially be used as a marker of perceptual decline in the course of Alzheimer’s disease.
The brain also possesses a strong capacity for object memory, yet evidence suggests that faces require a unique form of processing, Bainbridge says. Humans are drawn to the geometry of faces from an early age: Research demonstrates that infants show a preference for an image of two dots above a third (which more closely mimics a face) compared to two dots below a third. Faces are also visually potent; another study found that photographs with faces were more memorable than those without faces, as measured by a computer game asking participants whether they had seen a given image before.
Evolutionary adaptiveness can help explain humans’ skill for face memory in particular. Faces that embody more emotion or threat are especially enduring, Bainbridge says. “It may be less important to know a familiar or unfamiliar place, because you can take time to explore,” Bainbridge says. “But it could be really important to pick out your friends and your enemies.”
