A study of the birth and psychiatric records of 1.75 million Danish people whose mothers were born between 1935 and 1978 has shown that having a parent or a sibling with schizophrenia confers a relative risk of between 7 and 9 for the disease [Mortensen ,P.B. et al. (1999) N. Engl J Med 340, 603-608 (Abstract); also see Editorial by Andreason on p. 645]. Birth in a city or large town, or birth in the months of February or March, also increased risk (relative risk 2.4 for those born in Copenhagen, and 1.11 for February or March birthday). Although the highest relative risk is clearly associated with a family history, and so presumably with genetic factors, by far the highest population-attributable risks were environmental (34.6% for place of birth and 10.5% for season of birth, compared with 5.5% for family history).
Comment: The results of this very large study are instructive, not just for schizophrenia but also for many other complex diseases. Even if heritability is high (for schizophrenia it is estimated to be about 40%), at a population level the importance of environmental factors may far outweigh genetic ones. This is not a reason for abandoning the search for genetic variants associated with such diseases, but in public health terms the impact of finding such genes will probably lie more in what they tell us about the underlying biochemistry and pathology of the disease, rather than in any possibilities for genetic testing.