Of particular note is the potential of twin studies to distinguish between different models for the covariation between an endophenotype and an outcome variable. Models for comorbidity such as those described by Klein and Riso66 and implemented statistically for twin data by Neale and Kendler67 (see also Rhee et al.68) are potentially very valuable in this context. Interest in an endophenotype is usually diminished if it is a consequence of the ADHD phenotype as opposed to a cause of it. A more promising possibility is that the endophenotype and the outcome variable share certain liability risk factors. Bivariate analyses of data from twins permit some resolution between these alternative hypotheses.
