Methodology of twin studies

Twin studies are the archetypical study designs in quantitative genetic investigations of anthropometrical, behavioral, and health traits. These designs aim to estimate the extent to which individual differences in complex traits or phenotypes result from genetic and environmental influences. This chapter describes how the heritability of a trait can be estimated using data collected from pairs of twins, including the principles and assumptions of the classical twin design and basic methodological considerations. In addition, twin designs can be extended in multiple ways in order to analyze categorical data, interrogate gene-environment interactions, or include data from siblings. Several such extensions are described in this chapter.