In a psychiatric ward I once came across a young black man who had grown up as the adopted son of white parents. He claimed to be a visitor from another planet who would one day be rescued by a flying saucer. Was he mad? Yes — at least in the way we generally think of madness — but it was easy to see where his irrational ideas had come from. Conventional psychiatry treats insanity as a disease of the brain, but after 20 years as a clinical psychologist I am convinced that what we call madness is often a logical, if exaggerated, response to real events.

The boundary between sanity and madness varies according to geography. Behaviour that might get you diagnosed with schizophrenia in Britain — hearing voices, let’s say — is regarded elsewhere as, if not normal, at least understandable. In Africa traditional healers are often prized for hearing voices as it demonstrates their contact with the spirit world.Research shows that perhaps as many as one in 10 Britons hears voices but most are scared to mention it.

The way madness manifests itself is also influenced by time and place. The most common type of delusion in Britain today is persecutory: sufferers think they are being watched or listened to and that unseen enemies are out to get them. In America during the great depression, patients admitted to psychiatric hospitals commonly suffered delusions of great wealth or special powers.

Some cultures suffer syndromes all their own. Koro afflicts Chinese people (usually men), who imagine their sexual organs are shrinking. Latah affects Indonesians, who develop an exaggerated startle response, shout rude words and mimic people nearby. Nobody knows why these variations occur but it is clear that environment plays a role when we begin to lose our grip on reality. The boy brought up by white parents may have felt particularly isolated, but British-born children of Afro-Caribbean immigrants in general suffer a higher than average incidence of psychosis. One can only speculate that a sense of rootlessness and racism conspires to cause instability.

What is often not taken into account by psychiatrists is that people suffering from a mental illness have actually suffered loss, abuse or trauma. British psychiatry took a wrong turn in the late 19th century when we began to believe that madness was a disease of the brain rather than a reaction to the world around us. It has led to psychiatric services that are coercive and have sometimes inflicted terrible, even life-threatening “treatments” on patients. For research I have often asked for an hour of a patient’s time, saying it will not do them any immediate good but will assist me in trying to work out where mental illness springs from. At the end of a session they will often say: “You’re the first person to take me seriously.”

In a modern psychiatric ward nobody treats patients as if they have problems, yet we know from research that being sexually abused as a child markedly increases your chances of hallucinations in adulthood. If someone has a history of victimisation it is not surprising if they build up a paranoid world view but, too often, nobody bothers to ask.

So people who don’t need them are taking large doses of harmful drugs and their problems remain unsolved. We need an overhaul of the way we think about mental illness. Maybe the way forward is not to fight madness but to accept it.

Madness Explained, by Richard Bentall, is published by Allen Lane on July 20