“Oh, you know, still Queen,” came the reply.

As identity confusion goes, Beecham’s was on the extreme side. My most humbling to date is inexplicable to the unenlightened. I arrived at a restaurant to meet the man I had been dating for a month. Seeing him alone at the bar holding a drink, I greeted him with a brief kiss. We talked for a few minutes before an embarrassing thought washed over me. “We’ve never met before, have we?” I asked. “I wouldn’t have said so,” he replied, “but you seemed pretty sure.”

This incident, along with several other humiliating experiences, led me to the realisation that I am face-blind. It is a condition called prosopagnosia – face ignorance. I am not blind to faces; when looking at one I can describe it flawlessly. It’s just that when I look away, facial features generalise. Fellow prosopagnosic David Fine, consultant physician at Southampton General Hospital, has a simple but evocative way to describe face blindness: “I tell colleagues it’s like dyslexia, but with faces instead of letters.”

Prosopagnosia (PA) was first described in 1947 by a German neurologist, Joachim Bodamer, who examined second-world-war soldiers with brain injuries. Several soldiers shared a common complaint: they could no longer recognise their recruits or the faces of the medical staff. Since his landmark paper, many studies of PA have been conducted in which the subjects acquired the disorder following brain trauma. This can be sustained in many ways: in accidents, following asphyxia at birth, and after gunshot wounds, viral diseases and exposure to carbon monoxide. In every case there was damage to the right hemisphere of the brain, and the potential for discovering the exact location of a face-processing centre began to intrigue neurologists.

But in 1976 a report was published in which a mother and daughter were both diagnosed with prosopagnosia. Neither had suffered head trauma, so was it possible that face blindness could occur in the absence of brain damage? And if so, did it run in families? It was not until 1999 that a second report of familial face blindness was documented – in a father and two of his four children. By this time it was becoming increasingly apparent that PA could be hereditary.

Data released this year by a team led by Ingo Kennerknecht at Münster University suggests that PA is often caused by a single faulty gene that is passed dominantly between generations. An affected parent passes it to approximately 50% of his or her children, and daughters and sons have an equal chance of inheriting it. With close corroborations from Dr Chris Chabris at Harvard, Kennerknecht showed that as much as 2.5% of the population could be suffering from this socially encumbering defect.

When I tell my friends about my PA, the inevitable retort is: “But you just recognised me!” Yes, because you’re wearing the same coat as you do every day, and you still have long brown hair and glasses. I knew a man at university who was 6ft 9in, always wore rowing Lycra, and had bright orange hair. I did not need to see his face to know who he was. Nobody did. It’s these clues that allow prosopagnosics to function relatively normally in society, but in situations where the clues are missing – for example, when uniforms are worn – they will flounder.

“The worst situation my face blindness put me in was when I entered the navy during the Vietnam war,” says Bill Choisser, a retired electrical engineer who coined the term “face blindness” after setting up an internet forum for as-yet-undiagnosed sufferers. “I found myself surrounded by hundreds of people who looked exactly alike. They all wore the same clothes and none of them had any hair. This was so freaky, I completely lost it after five days. The psych team decided they didn’t want me in the navy.”

In contrast, the London-based architect Dominic Harris benefits from the liberal dress code in his office. “Clothing’s a really big clue for me. I work in a design studio and by chance it’s a really good profession for me, as people tend to express themselves through clothes and so on.”

But it’s not always so simple. “Where I really suffer is at round-table meetings with contractors,” continues Harris. “Everyone’s in a suit and blue shirt. I get my sketchbook and write a quick layout plan with arrows pointing at people, but they get up, then sit in a different place – it’s terrible when people move around.”

If you think you’re not prosopagnosic, here’s a way to get an idea of what it’s like. Buy a celebrity magazine, turn it upside down, open it but don’t look at the names, just the faces. After a while you will probably recognise the people, but it will take longer than usual. Studies have shown that face-recognition time is significantly slower when the face is presented upside down: those with normal face-recognition time take nearly the same amount of time to recognise faces upside down as prosopagnosics do when they’re the right way up. Interestingly, some prosopagnosics are faster at recognising upside-down faces and parts of faces than non-sufferers can, including perhaps Lewis Carroll’s intriguingly impaired Humpty Dumpty. In Alice through the Looking Glass, he suggests to Alice that life would be easier if her eyes, nose and mouth were upside down.

But when everyone is the correct way up, how do we identify those around us? Location can be both the greatest clue to someone’s identity and the reddest of herrings. I have only ever worked in small companies, where there are a limited number of options for who a person could be, but the assumption that you only find people where they “belong” can be misleading. I ran into a short man in my university’s management department once and assumed he was a friend of mine who was doing management training there. I was humiliated when I realised that not only was it someone else, but that I knew him too – as my friend’s flatmate. I had thought the two of them were one person for some six months.