It is not widely recognized that factor analysis of data from relatives confers several advantages over factor analysis from unrelated individuals. One benefit is that it is possible to analyze data from structured interviews, in which certain questions are asked if and only if a previous item has been responded to affirmatively. For example, in substance-use research it is possible to factor analyze jointly the initiation of substance use and subsequent symptoms of abuse or dependence.64 In the context of developing ADHD endophenotypes, it is possible to analyze commission errors for contributing characteristics and to ascertain how much they reflect biological characteristics specific to ADHD and how much they may reflect more generalized cognitive deficits.62 Second, when data have been collected from relatives and the factors correlate between relatives, it becomes possible to identify a greater number of factors than is the case with data from unrelated individuals. The data to identify a larger number of factors come from the cross-relative, cross-phenotype correlations. Third, given a genetically informative design, it is possible to partition variation in the latent factors into
