The sample consisted of parent and twin offspring (N ~4,900) who were part of the Minnesota Twin Family Study (MTFS). Our use of nuclear twin families made possible the investigation of the molecular-genetic basis of the endophenotypes in the same sample from which the biometric heritability of each could be estimated. There was very wide variation in the biometric estimates of heritability, from very high to essentially zero, which provided important context to evaluate the molecular-genetic results for each endophenotype individually and for the entire set considered together.
