Figure 1.

A hypothetical distribution of individual differences in age of onset for schizophrenia across 10 twin pairs. Solid bars indicate age of onset for each individual. Concordance is determined by the proportion of the 10 twin pairs who are both afflicted. Because individual differences in age of onset are observed for almost all forms of psychopathology, concordance rates necessarily increase across the life span. Note that the final concordance rate of .80 indicates a highly heritable trait. Although dichotomous outcomes are used for simplicity of presentation, the same argument applies to continuously assessed traits.