The inclusion of measured covariates in the Neale and Kendler model fitting approach.

Neale and Kendler (1995, Am J Hum Genet 57:935-953) described 13 comorbidity models, providing the most comprehensive set of alternative hypotheses regarding the possible causes of comorbidity to date. This research note describes an extension of the Neale and Kendler model fitting approach that permits the inclusion of measured covariates. An example of 13 models examining the comorbidity between alcohol dependence and illicit drug dependence is presented.