When information about a particular environment of interest is measured, the basic twin model can be extended to test whether the relative importance of latent genetic and environmental influences varies as a function of that environment. This can be accomplished either using a multiple group model, allowing one to test for differences in genetic and/or environmental influences across different groups (e.g., urban vs. rural environments), or through a subsequent extension of the twin model (Dick, Rose, Viken, Kaprio, & Koskenvuo, 2001; Purcell, 2002) allowing one to test for changes in genetic and environmental influences as a function of a more continuous environmental measure (e.g., scores on a family environment scale). In latent genetic studies, it is important to note that no specific genes are actually measured. Rather, genetic influences are inferred by comparisons of relative pairs. This provides an overview of the total aggregate genetic effect, and how that total genetic effect may differ across different environments, but it tells us nothing about the specific genes involved or the underlying biology. It has been suggested that now that we have
