In the classical twin model, phenotypic variance is decomposed into genetic and environmental variance components, which are usually assumed to be homoskedastic, i.e., constant across relevant environmental or genetic conditions. Heteroskedasticity will arise if the genetic and/or environmental variance components vary in size as a function of a given variable, or moderator. Such a moderator can be truly environmental in nature (e.g., exposure to radiation from a nuclear plant, the level of iodine in soil or drinking water1), or be a trait that itself is subject to genetic influences (e.g., eating or exercise habits, educational attainment level, personality traits). If moderators have a limited number of levels, their effects can be modelled in a multi-group design. However, a multi-group approach does not naturally account for group order, and quickly becomes impractical if the moderator is characterized by many levels (i.e., continuous in the extreme case). As few as, say, 3 or 4 levels may already require a challenging number of groups, especially if the moderator differs within twin pairs (i.e., is not ‘shared’), and the sample includes additional family members
