Our daughter is thinking of getting a T-shirt printed, with “Don’t blame me — I’m prosopagnosic” on it. Come to think of it, she could do a bulk order and include her mother, father and grandmother. The T-shirt might be able to get us out of a lot of tricky social situations.

Here’s a classic example. I’m sidling along a row in a theatre towards my seat at the centre of the stalls. Coming in the other direction along the row towards me, and clearly heading for the seat next to me, is a smiling woman. Do I know her? I guess I must do, though I’m not entirely sure. In my job, with a picture byline, I sometimes get greeted by total strangers. I am lacking a crucial clue: context. In a neutral venue, such as a theatre, I can’t tell if she is a colleague, a fellow parent at one of my children’s schools, a politician or a friend. So I put on my glassy smile and hope desperately that a few moments’ conversation — even the timbre of her voice — will identify her. Sure enough, it does. I soon realise that she is The Times’s managing editor, who sits in an office just a few feet from mine, and whom I see most days of the week.

Embarrassed? You bet! But this is an everyday occurrence for me, members of my family and millions of other prosopagnosics. What we have in common is that we are face-blind. We find it surprisingly difficult to distinguish one person from another. For me the distinction is hardest between people who have regular, symmetrical features and no particularly odd ones, such as a big nose or bad skin. Into this category falls my managing editor, as well as most actors, which is why I can’t tell the difference between Tom Cruise and Brad Pitt.

I have had this problem all my life. I can recall reading adventure stories in which the children would describe the baddie to the police, and thinking that I wouldn’t know where to start. I consoled myself that, as with most skills, I would get better at it as I got older. Sadly, I never did. My brother remembers, when we were teenagers, we watched a film with Paul Newman and Steve McQueen. Afterwards he asked me which was which. “I’ve no idea!” I confessed. Both were good-looking men with chiselled faces and blue eyes. How was I supposed to tell them apart? Prosopagnosics often find it hard to follow the plot of a film.

I hate it when there are lots of characters and particularly if they are all dressed alike. I found Gosford Park impossible: all the men had identical, slicked-back hair and dinner jackets, while the women had very similar hairstyles and dresses. I kept whispering to my husband, “Who’s that?” to which he would reply “God knows!” As you can imagine, we’re a hopeless couple at parties.

Like many prosopagnosics, though, I have developed ways of coping. I rely strongly on hair, glasses, voice, clothes and gait — which means I can be completely thrown if someone I know has a haircut, grows a beard or changes their specs. I try very hard to remember faces, but with little success. I often find myself spending an hour and a half having lunch with a new acquaintance, furiously trying to imprint his features on my memory. Yet the chances are that the next time I bump into him, I won’t know who he is. In fact, I could easily walk past him in the street the next day and not know that we had ever met.

The consolation is that I have at last discovered that the problem is not my fault. I used to think that I must be lazy or thoughtless or uninterested in other people. Now that I have taken part in a research study at University College London, I know that I am a proper prosopagnosic — and so are one of my daughters, my husband and my mother. My grandfather probably was too, but sadly it is too late to test him now.

My daughter Evie and I discovered our official prosopagnosic status before our results were even analysed. One of our computer tests had a man’s face on the top of the screen (shorn of hair — damn!) and what looked like exactly the same face replicated four times beneath it. We were then asked to rank the four “identical” faces in order of similarity to the one above. Simultaneously, we burst into giggles — for how could we rank faces that all looked the same? “I can always tell the genuine prosopagnosics,” said the researcher who was conducting the test. “All they can do is laugh when we give them this one.”

In the tests, Dai (my husband) and I came out as moderately prosopagnosic, while my mother and Evie were severely so. In most of the tests, my mother and Evie scored less than the bottom 1 per cent of the population. Dai and I did too in a few of them, but in others we were around the bottom 20 per cent. It may be that we used better strategies to compensate for our face-blindness. For instance, I ended up ranking the apparently identical faces according to the bushiness of their eyebrows.

We were all pretty bad at spotting famous faces. Without their hair, I even had to think twice before identifying Tony Blair and the Queen.

Brad Duchaine, the Harvard academic who is leading the UCL study, had sneakily put his own face into the mix. Although we had met and chatted with him just minutes before, none of us could identify his picture. “Is it that small American actor — you know, the Scientologist?” asked Dai.

I answered “Tom Cruise” to every dark-haired, regular-featured actor, reckoning that I’d be right at least once.

Duchaine believes that different parts of the brain are involved with processing faces and objects. Sure enough, his research demonstrates that for most prosopagnosics the difficulty arises only in discriminating between faces, and not other similar objects. The computer flashes up ten faces, for a couple of seconds each, twice. Then it flashes 50 faces, among which are the ten you have already seen. You have to say whether each face is new or whether you have seen it before. The test is then repeated with houses, horses and cars.

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