Introducing yourself to people you've already met is pretty embarrassing, especially when those people aren't just acquaintances but your best friends or immediate family.

Such is the case for those suffering from prosopagnosia, or face-blindness, a severe deficit that makes it difficult to distinguish between faces. Common horror stories include picking up the wrong child from daycare, kicking out of bed a "stranger" who is actually a spouse and, in the most extreme cases, failing to recognize oneself in the mirror. Even watching movies can be a burden: Too many faces make it impossible to keep track of the characters.

Bronwyn Carlton, a prosopagnosic living in New York, says she can see and even draw faces, but she forgets the image once a person steps away. She marvels at people who can instantly identify others by face alone. "I can't imagine what that's like," she says.

Until recently, only a few hundred cases had been documented. But using an Internet-based diagnostic test, researchers at Harvard University and University College London now estimate that up to 2 percent of us live with some degree of the disorder. That figure corresponds with another estimate made by researchers in Germany.

Prosopagnosia develops in two ways. With acquired prosopagnosia, the more common form, individuals become aware of the problem shortly after brain injury or stroke, often later in life.

Developmental prosopagnosia, on the other hand, appears early and occurs without brain damage. Comparable to the color-blind, developmental prosopagnosics don't realize they have the condition until tested. But testing is unlikely, given that "there is almost no awareness in the medical community and the public at large that such a deficit exists," says Ken Nakayama, one of the Harvard researchers.

A gene for face-blindness has not been identified, but the condition may run in families. Nakayama says 20 percent of his survey respondents reported having family members with similar problems.

No therapies exist to improve recognition, so prosopagnosics must rely on non-facial cues to identify people. For every person she encounters, Carlton memorizes a "police-blotter-type" description that includes height, weight, hair, and voice. Eyebrow rings, tattoos and scars are useful too. But such a strategy has its limits: People cut their hair and remove piercings.

Test your skills at: icn.ucl.ac.uk/facetests

Glenn, 28, of Brookline, Massachusetts

Cause

Unclear, though as a baby a tumble from his crib put him in a coma for six weeks.

First Prosopagnosic Memory

At age 5, Glenn bit a bully for teasing him. When confronted by his teacher, Glenn wasn't sure that he bit the right classmate.

Early Signs

When lost, Glenn would ask strangers, "Are you my mommy?"

Romantic Partners

He prioritizes a "quirky appearance"—which makes partners more identifiable—over physical beauty and still debates when, or even whether, to disclose his condition.

TV and Movies

Glenn prefers TV shows with easily distinguishable characters, such as Star Trek, and movies with one central character such as Forrest Gump.

Why We Used A Pseudonym

He fears criminals may target him because of his inability to identify them.

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