There is no doubt that the condition has a genetic aspect. Twin studies provide some of the strongest evidence, especially those comparing identical twins, who have entirely the same genes, with non-identical twins, who share only 50 per cent of their genetic material, and are no more alike genetically than two siblings from different pregnancies. If one twin has schizophrenia, about 50 per cent of the co-twins also have schizophrenia if they are identical, but only about 10 per cent if they are non-identical. Three points follow:

• There must be a genetic component of the causation of schizophrenia; otherwise, there is no way of explaining the differences in the 'concordance rates' between identical and non-identical twins.

• However, the condition is not completely genetic; otherwise, there would be 100 per cent concordance between identical twins for the condition.

• Therefore, environmental factors must also be important. Hunting the gene for schizophrenia

The human genome project was trumpeted by its projectors as the fundamental answer to medical mysteries such as the causation of disease. Unravelling the DNA would thus lead to the discovery of the underlying biochemical abnormalities that, they presumed, were responsible for otherwise mysterious conditions such as schizophrenia. A new discipline of 'psychopharmacogenetics' (Tsapakis et al., 2004) was even suggested, whereby genes would in some way guide drug treatment.

Unfortunately, like other reductionist movements in medicine, the so-called human genome project, even after the expenditure of enormous sums of money, has not led to major advances in understanding in medicine in general, let alone to practical advances for psychiatric patients. It has recently been decided that the answer in respect of schizophrenia, at least, is to sequence the entire human genome again, but this time in patients with the condition (http://www.; one hopes this will not be throwing good money after bad.

The overall pattern for the condition is that a first-degree relative of a patient with schizophrenia has an approximately 10 per cent chance of having the disease himself, as compared with approximately 1 per cent in the general population. Second-degree relatives have about a 3 per cent risk. If both the parents have schizophrenia, the risk in the children is about 50 per cent. Even if children with a genetic risk of schizophrenia are reared away from their biological families, as through adoption, there is not much reduction in their risk of getting the condition, confirming a genetic component in its aetiology.

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