Individual variation in the trajectory of brain development arises from the interaction of genetic and environmental factors. Statistical quantitative genetics, developed by the work of Fisher, Wright, and others (Fisher 1918; Wright 1968) provides a method of estimating the relative contributions of these factors based on the similarity of a given trait within individuals of different degrees of relatedness. In this model, phenotypic variation can be expressed as the addition of genetic, shared environmental, and unique environmental factors, plus their interactions. Heritability, or h, is defined as fraction of total variation due to genetic factors. Statistical quantitative genetics provides a broad estimation of what types of factors affect variation in a trait, without knowing what the specific relevant genetic or environmental factors actually are, particularly useful in complex traits to which it is likely a large number of genetic or environmental factors may each be contributing a small component. One of the most commonly used and powerful study designs within quantitative genetics is the twin study.
