Reports that the Yorkshire Ripper, Peter Sutcliffe, may appeal against his conviction on the ground of diminished responsibility because he had paranoid schizophrenia when he murdered his victims can only reinforce public perception that the mentally ill pose a serious threat and further stigmatise an already poorly understood condition.

Some people with serious mental illnesses are dangerous but, although cases involving them invariably hit the headlines, they represent a tiny minority of the hundreds of thousands affected. Even when a person’s illness does lead to violence, it is nearly always directed against him or herself. People with schizophrenia are much more likely to take their own life than to harm others.

Despite major strides forward in our knowledge of the brain, schizophrenia remains much less well understood than conditions affecting other vital organs in the body. It is a striking illness, though, because it is so alien to the uninitiated. I vividly recall meeting my first case in a cell-like room in a grim asylum. I was a medical student and he was a young man of similar age to me who had been placed in hospital against his will. His problems had started when he withdrew from his college course and barricaded himself in his digs to keep out the “watchers” — people who had been monitoring him and his thoughts since he began university. But the barricade hadn’t stopped them from inserting a bug into his brain so they could eavesdrop. I can still see the earnest look on his face as he pulled me close to whisper that he knew the psychiatrist didn’t believe him, but he had proof that the bug was there because he could smell hot electrics in his sweat.

Schizophrenia can present in a number of ways but delusions such as this — fixed, irrational and unshakeable beliefs — are common and, along with hallucinations such as hearing voices, are among its most striking features. The condition is thought to result from a combination of genetic predisposition and exposure to environmental factors: Nature loads the gun, the environment pulls the trigger.

Recognised triggers include drug abuse (cannabis is associated with a 40 per cent increase in risk), stress and traumatic life experiences. There is also growing interest in the role of infection. Studies have indicated that pregnant women who catch flu in the first half of their pregnancy are three times more likely to give birth to a child who later develops schizophrenia. Other research suggests that infection with the parasite toxoplasma (caught through contact with cat faeces and undercooked meats and carried by at least a fifth of the population) may trigger the illness in some susceptible individuals.

Schizophrenia typically develops in teenagers and young adults, and florid symptoms are often preceded by a period of social withdrawal and odd behaviour. The earlier it is diagnosed and treated, the better the outcome. Women generally do better than men, and the older you are when the illness is diagnosed, the less debilitating it is likely to be. The outlook isn’t always bleak. One person in five who requires admission with schizophrenia will never have another acute episode. However, half will relapse intermittently throughout their lives, and about a third will have a persisting debilitating illness.

The psychiatrist must try to ensure that those affected lead as near normal a life as possible. But it is an unpredictable business and people make mistakes. Patients stop taking their medication and slip through follow-up procedures, while others may not be picked up early enough to prevent catastrophic consequences — all too often their suicide.

Although a tiny minority are a threat to society, it is the families of the seriously mentally ill who suffer most. During my brief stint in psychiatry and 20 years in general practice I have never had a mentally ill patient who has seriously harmed anyone — but I have had plenty whose illnesses have wrecked the lives of their families. This suffering is unlikely to be helped by the belief that the seriously mentally ill are “nutters” who need to be locked up to protect the rest of us. Sutcliffe, however, is a notable exception.

• There are half a million people with schizophrenia in the UK, half of whom will attempt suicide. At least one in ten succeeds.

• Recent research suggests that people with schizophrenia are just over twice as likely to commit a violent crime as the rest of the population, but much of that additional risk is in those abusing drugs.

• If you have one parent with schizophrenia you are ten times more likely to develop the condition. If both parents have it, there is a 50 per cent chance.

Dr Mark Porter