Have you ever bumped into someone with a face that feels familiar, but you just can't quite narrow down why you recognize them, or if you really recognize them at all?
For some people with prosopagnosia, also called "face blindness," that confusion is a regular, daily occurrence.
But what is prosopagnosia, and what causes it in the first place?
What Is Prosopagnosia?
According to the National Institute of Neurological Disorders and Stroke, prosopagnosia is a neurological disorder that causes someone to have trouble recognizing people they know, or distinguishing between different people's faces.
Also called "face blindness" or facial agnosia, it has different levels of severity for different people.
Some people only have trouble recognizing the owner of familiar faces. Someone with prosopagnosia may not register someone they've known for years when they see them at the grocery store or may have trouble recognizing their own face in a photo or the mirror.
The symptoms of face blindness may also mean a person with the disorder has trouble figuring out what age or gender someone is, and they may not be able to tell what expression that person is making, according to the U.K.'s National Health Service.
Other people may "be unable to discriminate between unknown faces, while still others may not even be able to distinguish a face as being different from an object," according to NINDS.
What Causes Face Blindness?
According to NINDS, the disorder is caused by differences in a part of the brain called the fusiform gyrus.
The fusiform gyrus is "a fold in the brain that appears to coordinate the neural systems that control facial perception and memory" -- in other words, the part of the brain that allows you to see a face and remember if, when and where you've seen it before.
That part of the brain is "a spindle-shaped fold in the inferior surface of your brain (it’s located in the temporal lobe behind your ears)," Kalanit Grill-Spector, chair of Stanford's department of psychology and an expert on face recognition who has conducted significant research on prosopagnosia, said in an email.
One way that "atypicalities" in that part of the brain may lead to face blindness, according to Grill-Spector, "is that neurons on this area process parts of the face but do not process the entire face as a whole, leading to problems in face recognition."
That means researchers believe prosopagnosia isn't related to memory loss or dysfunction, or to vision impairment.
"It is not a vision problem because people with prosopagnosia can see perfectly fine and recognize other objects like cars or places perfectly," Grill-Spector said in an email. "It is not a memory problem because they can recognize people from other attributes like their voice or name and they have no general problem remembering things or details."
For some people, the disorder "can result from stroke, traumatic brain injury, or certain neurodegenerative diseases," NINDS said.
Those people have what's known as acquired prosopagnosia, where the condition develops after some type of damage to the brain.
Other people have prosopagnosia from birth, without any brain damage occurring. In those cases, it's known as developmental prosopagnosia.
That version of the disorder may have a genetic component, with the NHS and NINDS reporting that it sometimes runs in families.
It's also important to remember that, even though it affects how people see things, prosopagnosia is "very different" than conditions that cause hallucinations, according to Grill-Spector.
"[Prosopagnosia] is a very specific disorder as it affects only your face perception and there are no alternations in how people perceive reality or mood," whereas psychiatric conditions and their associated hallucinations have other effects.
How Common Is Prosopagnosia?
Although face blindness is relatively rare, it's still more common than you might expect.
A 2006 study from Harvard and University College London found that prosopagnosia may affect up to 2% of the population, meaning there may be millions of face-blind people.
"Testing of 1,600 individuals found that 2 percent of the general public may have face-blindness and a German group has recently made a similar estimate," Ken Nakayama, one of the lead researchers from Harvard for that study, said shortly after its release.
Grill-Spector affirmed that 2% estimate.
According to the NHS, "several studies have indicated that as many as 1 in 50 people may have developmental prosopagnosia."
If that statistic holds true worldwide, that amounts to around 6.6 million people in the U.S. with prosopagnosia.
"It’s conceivable that millions of people may have symptoms consistent with prosopagnosia, without even realizing it," Nakayama said in 2006.
What Is It Like to Be Face Blind?
People with prosopagnosia often have to rely on cues other than someone's facial features, "such as hairstyle, clothing, voice, or gait," to identify the people they know, according to researchers at Stanford.
"For typical observers face recognition is based on the whole face (for example we are better [at] recognizing someone’s nose in the context of their face than in isolation)," said Grill-Spector in an email. "But for prosopagnostics is it the opposite - they are more focused on isolated features."
And because those facial features can look very different at different angles or in different lighting, face recognition can be very difficult for prosopanostics, she said.
"It is very confusing not being able to recognize people around you especially when this feat seems trivial to everyone else," Grill-Spector said. "For some people this causes social isolation and for other people their reaction is to be extremely outgoing to not insult people that they are supposed to know but can’t recognize."
It may also lead individuals with the condition to worry that they seem rude or bored when they don't recognize someone they know, the NHS said.
That's the experience Brad Pitt has, according to his GQ interview.
"He fears it’s led to a certain impression of him: that he’s remote and aloof, inaccessible, self-absorbed. But the truth is, he wants to remember the people he meets and he’s ashamed that he can’t,” the profile reads.
"So many people hate me because they think I’m disrespecting them,” he said in that Esquire interview. “Every now and then, someone will give me context, and I’ll say, ‘Thank you for helping me.’ But I piss more people off."
"You get this thing, like, ‘You’re being egotistical. You’re being conceited.’ But it’s a mystery to me, man. I can’t grasp a face and yet I come from such a design/aesthetic point of view. I am going to get it tested."
If Brad Pitt is interested, Stanford is ready and willing to scan his brain, Grill-Spector said.
"We’re happy to invite Brad Pitt to our MRI scanner at Stanford Psychology to map his face area," she said.
If you think you may have prosopagnosia, Grill-Spector said, this website has a list of common symptoms.
And if you are face blind, you're in good company. Other famous people with face blindness may include Jane Goodall and neurologist and science writer Oliver Sacks.