Distant voices

The fact that there are huge differences in rates of schizophrenia between racial groups, classes and urban/ rural populations is very unlikely to be due to genes. Indeed, it is powerful evidence for the role of environment.

Schizophrenia is three times more frequent in American Afro-Caribbeans and up to 16 times more common in children of West Indian immigrants to Britain. Whether it be Surinamese moving to the Netherlands, East Africans to Sweden, German immigrants or even Jamaicans returning home after a period in Britain, the rates are similarly increased.

Psychiatric theory assumes that these differences are primarily caused by genes, but at least in the case of British Afro-Caribbeans we can rule them out. Caribbeans who are born and raised there have the same prevalence of the illness as that found in most of the world - less than 1 per cent. When rates are studied among those who continued to live in the Caribbean but had relatives who emigrated to Britain and became ill, there is no increased risk among the stay-at-homes.

To top it all, rates are highest among the children of immigrants rather than first-generation ones. Taken altogether, this would seem to prove beyond any doubt that it is the environmental change of emigration that is the cause.

A recent study provided more than a clue as to why. Non-white people living in white neighbourhoods were more likely to become ill than ones living in non-white areas. Alongside racism, there are huge pressures on Afro-Caribbean parents, leading to extended periods apart from their children. Of 38 Afro-Caribbeans who had become ill in one study, a third had been separated for more than four years from their mothers during childhood, half from their fathers.

Poor people are about twice as likely as the rich to suffer schizophrenia. In 1939, American sociologists showed that the nearer you lived to the slums at the centre of Chicago and the poorer you were, the greater your risk. Equally, a survey of 2m Danes found that the more of your childhood you had spent in urban environments, the greater the risk. An international study even showed that the kind of symptoms you have depend on this. In rural areas, symptoms like loss of interest in appearance and cleanliness were commonest, whereas in cities you were more likely to hear voices and fear persecution - hardly likely to be anything to do with genes.

Contrary to the frequent claims of psychiatrists, rates of schizophrenia vary considerably around the world, as does its course. It is less common in developing nations and the illness tends to last longer and to be more severe in rich, industrialised nations compared with poor, developing ones. In fact, if you go mad in a developing nation you are 10 times less likely to have any recurrence of the illness - a huge difference, also nothing to do with genes.