Similarly to our analysis with heterogeneity by sex, we defined separate variance components for individuals whose age was greater or equal than the sample median (42 y of age), σg,old 2 and σe,old 2, and for individuals whose age was below the sample median, σg,young 2 and σe,young 2. When there was evidence for heterogeneity, we considered intermediate models in which heterogeneity was allowed only for environmental effects (i.e., where σg,young 2 = σg,old 2), or only for genetic effects (i.e., where σe,young 2 = σe,old 2), or in which the total variance for both genetic and environmental factors changed by a shared factor (σg,young 2 = k σg,old 2 and σe,young 2 = k σe,old 2, with k ≠ 1).
