Once, in broad daylight, Bill Choisser encountered his mother on a sidewalk in a local shopping district. He walked toward her and passed her within two feet. He said nothing as he ambled by--an omission for which his mother has never forgiven him, or so he writes in his online book, Face Blind! Choisser meant no ill will toward his mother, however; he simply did not recognize the woman who raised him.

Many people have trouble remembering names. Choisser cannot remember faces, a condition known as prosopagnosia. People like Choisser can see faces, but they cannot keep in mind what particular people look like. The fact that all visages contain basic features such as a nose, mouth and eyes makes every face appear essentially the same to them, like so many stones paving a gravel driveway. As a result, features hold just about as much fascination for them as ridges on a rock.

Until very recently, such selective face blindness was thought to be extremely rare, with perhaps 100 documented cases worldwide--and most of them people who had acquired the condition as a result of head trauma, stroke or brain disease. In the past few years, however, my wife, Martina Grueter, and I, along with her colleagues at the Institute of Genetics in Muenster, Germany, have revealed that a surprising number of people are born with prosopagnosia.

Our surveys, along with those conducted by cognitive neuroscientists Ken Nakayama of Harvard University and Brad Duchaine of University College London, indicate that this perceptual quirk resides in 2 to 3 percent of people. If this is true, up to six million prosopagnosics reside in the U.S. alone. Many of them do not realize that their perception is abnormal, because they have always been this way--perhaps explaining why the disorder has gone unrecognized for so long. Prosopagnosia also seems to run in families, as our recent work shows, and may be caused by a change in a single gene.

Given the importance of faces in human interactions, a difficulty in perceiving them can be socially and professionally crippling, causing embarrassment, confusion or worse when a person fails to recognize a close friend, family member, customer, boss or colleague. Some prosopagnosics attempt to avoid casual encounters in public places because they fear humiliation. Most use various tactics for hiding their deficit. And although individuals with prosopagnosia can glean emotional and other clues from faces, they often do not, because they fail to look at a person's countenance--its features are so unimportant to them.

In young children, prosopagnosia can cause excessive clinginess to parents, according to our ongoing investigation at the University of Vienna in Austria. Children with face blindness also may require months to learn to recognize their school classmates, making school transitions difficult. Variations in the appearance of others--such as a new beard or a pair of glasses--can also be disconcerting to affected kids because, we have found, such changes disrupt their fragile memories of those faces. Once identified, children with prosopagnosia can be given practical pointers and assistance that enable them to better cope with their deficit and thereby decrease their feelings of frustration, embarrassment and isolation.

Short Circuit
Face processing occurs in many regions of the brain, but imaging studies indicate that the so-called fusiform face area, a subdivision in the brain's temporal lobes--large sections of the outer brain that sit over the ears--is especially important. The occipital face area near the back of the brain may also play a role in categorizing an object as a face. And the superior temporal sulcus, also in the temporal lobe, has the job of responding to changes in a person's expression or viewing angle [see box on opposite page].

The notion that a short circuit could occur selectively in the brain's face-recognition machinery emerged in the 1940s, when a German neurologist, Joachim Bodamer of the Winnental Sanatorium near Stuttgart, examined two patients who had sustained severe head injuries in World War II. Bodamer noted that although the men could still see faces, they could no longer place them. He concluded that seeing and recognizing faces represent different brain functions. In a 1947 paper Bodamer coined the term "prosop-agnosia" from the Greek prosopon, or face, and agnosia, meaning nonrecognition or without knowledge.

Over the next several years, physicians noted the same phenomenon in victims of stroke and other neurological damage, particularly when brain tissue had been destroyed at the transition between the occipital and temporal lobes. The first case of congenital prosopagnosia, however, did not appear in the medical literature until 1976: a girl who recognized her school classmates only after studying their voices and clothing for a month or two. And over the next 25 years, only about a dozen more such cases came to light.

Loss of Face
I have prosopagnosia. I easily overlook people I know in public spaces such as airports or shopping malls. I do not define people by their facial features per se but by a patchwork of other characteristics such as their typical expression, gait and voice. I have always known that I had a deficit in facial recognition, but it never occurred to me that it might be a medical condition until 2001, when Martina happened to see a TV show on prosopagnosia. I suddenly realized that its symptoms matched my own.

So I then asked a doctor at the Institute of Genetics in Muenster if an inability to recognize faces was a known congenital condition. He replied that it was not. Still curious, however, Martina typed the German term for "face blindness" into Google and quickly turned up two German women who reported problems recognizing faces. From there, she tracked down more candidates by posting queries on relevant Web forums and thereby launched a study that would become her M.D. thesis.

Ninety people agreed to be interviewed for signs of the disorder, and she found clear evidence of prosopagnosia in 31 of them, a number we found hard to believe in light of the paucity of known cases. But we confirmed our diagnoses using face-recognition tests and repeat interviews conducted by specialists from Cardiff University in Wales. The affected subjects were clustered in six families, including our own, which further suggested the existence of an inherited disorder, Martina reported in 2004.

Convinced that we were onto something, Martina and I, along with her Institute of Genetics colleagues, then probed the prevalence of prosopagnosia in the general population. We gave written questionnaires to students from three local secondary schools as well as medical students from the University of Muenster, and we selected for follow-up questioning the 10 percent of respondents whose answers hinted they might have the disorder.

We interviewed these subjects to determine, for example, whether they lacked confidence in recognizing people; had prepared excuses when greeted by somebody they could not identify (an indication that they frequently find themselves in this predicament); or often tried to avoid people in public places. Because of the awkwardness of being unable to recognize someone who knows you, face-blind individuals will often try to steer clear of chance encounters by, say, crossing the street or pretending to talk on a cell phone when they see someone approaching.

Most of those we interviewed, however, showed no abnormalities. (A number of them were simply shy.) But 17--or 2.5 percent of the 689 students surveyed--met our criteria for prosopagnosia. We then interviewed the close relatives of the 14 affected pupils who agreed to participate, and we discovered that every one of their families contained other members with prosopagnosia.

Our results, which we reported in 2006 in the American Journal of Medical Genetics, suggest that if a parent displays this perceptual weakness, a child stands a 50 percent chance of exhibiting it, too. An alteration in a single dominant gene could account for this pattern of transmission; only one copy of such a gene--from a mother or father--is required to transmit the trait.

By then, we were not the only ones hot on the trail of face blindness. In a 2006 Harvard press release Nakayama and Duchaine reported surveying 1,600 participants on the Internet for the presence of prosopagnosia using a series of face-recognition tests. Concordant with our results, about 2 percent of their subjects had serious difficulty recognizing faces.

Are You My Mother?
A tendency to pay attention to faces is present from birth, but such perception does not completely mature until the teenage years. Nevertheless, we assumed that the effects of an inherited disorder would show up early. So last year my wife and I, now at the University of Vienna, began investigating the impact of prosopagnosia on children by interviewing the mothers of boys and girls diagnosed with the condition.

As babies and toddlers, the mothers told us, such children tend to be unusually anxious whenever their mother leaves the room and often object when their grandparents pick them up. Although wariness of strangers is normal in an eight- to 12-month-old child, this apprehension seems to linger much longer in children with prosopagnosia. "It was torture for me whenever the toddler group got together," one mother recalled. "All the other children played happily together except for Stephanie, who sat anxiously on my lap and refused to get involved." Because she could not reliably recognize her mother, Stephanie feared that she would not be able to find her mother again if she separated from her.

As with other children who are face blind, Stephanie required a long time to get used to the playgroup and to be able to recognize individual playmates. In many cases we heard about, a toddler had been unable to tell whom he or she had played or fought with half an hour earlier. The mothers also noted that their face-blind children would often become irritated when their mom changed her hairstyle or wrapped her head in a towel after bathing. Any such variation can disrupt the child's tenuous memory of the mother's appearance and thus make her more difficult to recognize.

The difficulties continue into elementary school. "It took Anna half a year to warm up to the other kids," one mother told us. Her teacher complained that Anna seemed unfocused in class because her gaze wandered. But Anna did well in school, and she explained that she could follow the teacher better if she did not have to look at anyone.

The disorder can cause logistical problems for kids, too. One mother, who is also affected, recalled a time in grade school when she mistakenly went to the wrong classroom after the room had been reassigned. Because she could not tell that the other children in the room were not her usual classmates, she remained in the wrong room until the teacher arrived and informed her of her mistake.

Teeth and Shoes
There are as yet no therapeutic solutions for prosopagnosia. Still, children with the deficit can be taught other ways to recognize people (among other coping techniques) [see box on opposite page]. These include memorizing what people wear and how they style their hair, as well as their gait and their speech patterns. By age 14, affected individuals have typically come up with such tactics on their own. One dentist told us she identifies people by looking at their teeth when they are smiling. Another prosopagnosic eyes men's shoes on the assumption that men own very few pairs.

Of course, such crutches do not work as well as a fully functioning face-recognition system. Thus, the deficit is persistently disabling, often in the most embarrassing ways. "I was having a wonderful conversation with a woman at a party, but then I went to get us some drinks," one young man sheepishly recalled. "When I returned, I had forgotten what she looked like, and I was unable to find her the rest of evening. She must've thought I was a complete idiot!"

More widespread understanding of the condition could ease much of this awkwardness. It also could have important practical payoffs. Screening eyewitnesses slated to testify in court for prosopagnosia could be beneficial, for example.

Meanwhile researchers are actively searching for the prosopagnosia gene. Finding this genetic quirk would not only advance the biological understanding of face perception but also enable doctors to diagnose the condition in very young children. And the earlier children can be properly identified as face blind, the better the chances that these kids will get the help they need to happily find their way in the world.