Before any molecular genetic study is undertaken, the methods of genetic epidemiology are used to identify a phenotype (observable disease or trait) that is at least partially heritable. An introduction to these methods is available online (http://www.dorak.info/epi/genetepi.html). Briefly, twin, family and population-based studies are used to estimate heritability, define the most heritable phenotype, and explore interactions between genetic and environmental factors. The current diagnostic definitions of major psychiatric disorders are based in part on twin and family data. Epidemiological data are also critical for defining appropriate control groups for molecular studies. The data for psychiatric disorders suggest that most of the heritable risk is due to interactions of combinations of genetic risk variants, each with a relatively small effect on risk.
