Many psychiatrists believe that schizophrenia's tendency to run in families implies that it is heritable. While less than one per cent of the general population are likely to get the illness in their lifetime, 17 per cent do so if one of their parents had it. The figure rises to 46 per cent if both parents did. But the view that just being raised by schizophrenics might be a cause of schizophrenia is rarely considered by psychiatrists - and usually actively discouraged by them.

The starting point for an environmental explanation of schizophrenia is uncontested, even by geneticists. There is a large body of evidence now proving that the course of the illness is hugely affected by the way families react to a patient when they are released from hospital.

In about half of families with a schizophrenic member, the parents are rather negative towards a child after he or she returns home following a breakdown. The parents may make a lot of critical comments, be hostile and get over-involved in trying to control their offspring. Whereas 55 per cent of patients who return to a negative home like this relapse back into schizophrenia, only 16 per cent do so if their parents are positive.

Compared to families with no presence of schizophrenia, studies of relationships in families with a schizophrenic child show that the parents tend to be more dominant - perhaps attempting to quell an impossibly difficult child - and are less respectful of the child's autonomy. They also tend to be more mystifying, perhaps because the parents are discombobulated by their child's weirdness.

But if negative parents are trained to be less so, the relapse rate of their children falls. When patients with negative parents are sent to a hostel instead of being sent home, they are less likely to relapse. Overall, parental reactions have far more effect on relapse rates than whether or not patients keep on taking their prescribed drugs.

This evidence tells us nothing, in itself, about the original reason the illness came about. It could be that the parents are negative because they have been stretched to the limit by an incredibly difficult child with a genetic ailment, and doubtless that is sometimes the case. But it could be that the parental negativity has been there since before the illness began and was, in fact, its cause. Both explanations could apply: a degree of genetic proneness to schizophrenia in the child could have meant that those parents who were ill-equipped to cope with such a worrying offspring became negative, thereby exacerbating the schizophrenia.

The critical question is whether or not the parental negativity predated the schizophrenia; several good studies strongly suggest that it often does... which is next week's subject.

· oliver.james@observer.co.uk. Oliver James is unable to enter into any personal correspondence