Mass killings by a lone gunman lead to speculation about the sanity – or lack of it – of the killer who often turns out to have expressed doubts about his own mental stability prior to the attack. For instance, James Holmes, who shot dead 12 people and wounded 58 as they watched The Dark Knight Rises in a cinema in Colorado in July, had sent a text to a friend two weeks earlier suggesting he might be suffering from a form of bipolar disorder, the symptoms of which include manic energy, rage, fearfulness and paranoid delusions.

Slaughter of innocent victims on a mass scale for no apparent reason, whether it takes place in the US, Norway or Britain, invariably attracts massive coverage, though usually of a crude and superficial kind. Stereotypes are reinforced by the hundreds of films similar to Psycho and The Silence of the Lambs showing a crazed but demonically cunning killer at work.

It is a pity that these killings, real and cinematic, never lead to any discussion about how the mentally ill have been treated and mistreated in the US and Britain over the past half century. This is despite the fact that so much of the population is affected directly or indirectly by psychiatric disorders.

An estimated one in six of the adult population of England and Wales, about seven million people, will suffer significant mental health problem at some point in their lives. If their families, whose lives are frequently devastated by the breakdown of one member, are included, then a big chunk of the population is involved.

Why isn't this talked about more? Fear is probably the main reason, because people are terrified of madness, or anything close to it, to a degree they no longer fear physical illnesses. I was amazed after my son Henry was diagnosed with schizophrenia (after trying to swim Newhaven estuary in the middle of winter in 2002), to learn how many of my close friends had mentally ill family members about whom they had never spoken.

By the nature of their disability, the mentally ill can rarely speak out for themselves. Combined with the reticence of those closest to them, this makes them uniquely voiceless and vulnerable. As a result, they have been the victims of one of the most regressive, cruel and culpable actions carried out by governments in the modern world during the last 50 years.

Over this period, without the public paying much attention, governments have abdicated their responsibilities to treat the mentally ill. In the US, the number of beds available for psychiatric hospitals dropped 90 per cent, from 558,000 in 1955 to 53,000 in 2005. In Britain, between the 1950s and today, the fall in the number of beds for psychiatric patients was from 150,000 to 30,000. People with mental disorders were casualties of a pincer movement from right and left. The right wanted to save money and lower taxes, while the left and liberals believed that mental patients had been unnecessarily incarcerated as social deviants whom the Victorians wanted to hide from the world. Drugs developed in the 1950s would make constant care unnecessary.

Ignored in all this was the fact that a significant number of people with mental illness do need asylum in the old sense of the world. They cannot look after themselves even with medication, which generally controls but does not cure, and the unrelenting nature of their illnesses means they are never going to get better.

There were upbeat but hypocritical justifications for what was happening. In the US, the running down of the public psychiatric service was sold as "deinstitutionalisation", supposedly ending unnecessary confinement. In practice, people who could not fend for themselves were thrown into the street and worse was to come.

A high proportion of homeless, vagrants and street people were mentally disturbed and bounced in and out of prison. America had effectively criminalised people with severe psychiatric illnesses. The Treatment Advocacy Centre (TAC), a US group that seeks to promote the interests of the mentally ill, wrote a chilling report showing that "jails and prisons have become, de facto, the nation's largest psychiatric hospitals. There are now more severely mentally ill individuals in the Los Angeles County Jail, Chicago's Cook County Jail, or New York's Rikers Island Jail than in any single psychiatric hospital in the nation." Nowhere in the US escaped the devastating impact of "deinstitutionalisation" on those it was purportedly helping. In every one of the 3,139 counties in the US, in 2009, more people with severe psychiatric disorders were held in jail than were being treated in local psychiatric facilities.

Conditions for the mentally disturbed in penal institutions are sometimes little better than they were in the 18th century when the miseries of the insane were painted by Goya and Hogarth. Bizarre behaviour by mentally ill prisoners can make them targets for violence by other prisoners. For instance, a study of prisons in New Jersey three years ago reported that mentally ill male prisoners were three times more likely to be raped than those who were not mentally ill.

Disturbed people living rough are likely to take drugs and alcohol and be repeatedly arrested for minor offences such as trespassing and disorderly conduct. To take one example cited by TAC, a woman with schizophrenia was arrested in New Mexico for assault after she rearranged the shelves in a shop where she had a delusional belief she was working. Asked to leave by the shop manager and the police, she struck out. In many cities, particularly those that rely on tourism such as New Orleans, local businesses demand that police arrest mentally ill street people behaving strangely who might deter potential customers.

Similar policies have been adopted in Britain and the rest of Europe. The network of mental hospitals built up by the Victorians was dissolved and the valuable land they were built on sold off. The hypocritical name of the new policy was "care in the community", better described by one Labour health minister as "couldn't-care-less in the community". The writer P D James, who worked as an administrator in the NHS, commented that it "could be described more accurately as the absence of care in a community still largely resentful or frightened of mental illness". In practice, it is the families of the mentally ill who have to take the strain.

Can the grotesque errors of past mental health policy be reversed? The Government and lobbying groups support a campaign to reduce the stigma attached to those suffering from mental illness. They argue that the psychiatrically disturbed are less violent than people imagine because of exaggerated media coverage.

The lobbying group Mind argues that the danger is limited since 95 per cent of homicides are committed by people without mental problems and there are only "50 to 70 cases of homicide a year involving people known to have a mental health problem at the time".