ABSTRACT
A genetic predisposition to schizophrenia may be present in as many as 7 to 10 per cent of the population. This is the assumption made by genetic researchers doing linkage studies (Wang et al., 1995; Freedman et al., 1997). Yet, as mentioned in the Introduction, the illness becomes manifest in no more than 1 per cent of the population (Warner and de Girolamo, 1995). Since only a fraction of those genetically at risk develop the illness, we have to assume that either it takes more than one gene to cause the illness or that the addition of an environmental factor is necessary. We know, in fact, that non-genetic, environmental factors are essential, because, despite being genetically indistinguishable, the identical twin of someone with schizophrenia has only a 50 per cent chance of developing the illness, not a 100 per cent chance (see Figure I.3). Preeminent among these causative environmental factors, it emerges, are complications of pregnancy and delivery.
