The study of genetic epidemiology is of interest because it seeks to attribute a relative weight to the effects of genes, the shared and non-shared environment, and gene-environment interactions. Sullivan and colleagues (2002) have recently reviewed the literature on the genetic epidemiology of major depression, using specific inclusion criteria. On the basis of the five family studies that met their criteria, they concluded that the odds ratio of being affected with major depression in the first-degree relatives of probands compared with those of unaffected comparison subjects was around 2.8.
Twin studies are capable of providing estimates for the relative strength of genetic effects and the effects of the shared and non-shared environments. Sullivan and colleagues (2002) reviewed five studies covering 21 000 individuals that met their criteria. Although the studies showed appreciable variation in the heritability of major depression, all reported significant heritability estimates, while the estimates for shared environmental effects were non-significant in all the studies. There were no consistent sex differences in the results.
Despite problems in aggregating data of this type from different studies, it did prove possible. The best-fitting model included only genetic and individual-specific environmental factors, with the former contributing about 37% of variance. Heritability may be particularly increased in depression with an early onset, and is almost certainly so in recurrent depression. It should be noted that, as calculated, heritability estimates would cover gene-environment interactions as well as purely genetic effects.
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