There is, as Judith Rapoport acknowledged, a little bit of OCD in most people. Who has not gone back into their house twice to check that the gas is off or all the windows are locked? In extreme cases, however, such behaviour becomes so obsessive that it interferes with normal life – a sufferer will not check the gas or locks once or twice, she observed, but “10, 20 or 100 times”.

Judith Rapoport’s patients included the talented 17-year old who overnight found himself unable to stop washing, so obsessed with the idea that he was dirty that he dropped out of school; the man who felt so sure that he had hit someone with his car that he would continually drive up and down the road looking for the body; and the woman who went to confession five times a day to ask forgiveness for the “terrible sin” of coughing during a public lecture.

Sufferers, she found, are often aware that their actions are ridiculous, but are beset by such powerful anxieties that they are unable to stop.

Judith Rapoport’s research revealed a number of facts about OCD that were not generally known, even among psychiatrists. She found that when sufferers sought her out for treatment, their refrain was almost always the same: “I thought I was the only one in the world with these crazy symptoms, and I didn’t want anybody to know about them.” As a result they became expert at concealment – so much so that OCD was widely assumed to be an uncommon disorder.

In fact, she revealed, about 3 million adults and 1 million children in the US were locked into some form of repetitious behaviour. In Britain it is estimated that around a million people suffer from OCD, while between one and four per cent of the population may experience symptoms at some point in their lives.

As a trainee psychiatrist Judith Rapoport had been taught that even before the onset of their illness sufferers invariably had “obsessional personalities” – often blamed on over-strict parenting in childhood.