One approach has been to search for endophenotypes, simpler characteristics that are associated with the upstream causes of a complex phenotype such as a psychiatric disorder but whose links to genetic factors are potentially easier to parse (Gottesman & Gould, 2003). Several requirements are necessary for a trait to be an effective endophenotype. It must be not only associated with the syndrome of interest but also present independent of disease state. In addition it should be present in healthy individuals at increased genetic risk for the disorder, and variance of the trait must be measurably affected by genetic differences, that is, it must be heritable. Heritability is broadly defined as the amount of variation due to genetic factors. It is worth recalling that the heritability of a trait refers only to the extent to which genetic factors affect variation in the trait, and not whether genes play a role in producing the trait itself.
