For the purposes of this review, we drew upon recent quantitative summaries of the heritabilities of the disorders of interest, if available. To identify relevant studies of phenotypes not previously summarized, we performed PubMed searches (e.g., “Alzheimer Disease [MeSH Terms] AND twins [MeSH Terms]”), and we searched reference lists of source and review articles. We selected reports that met modern methodological standards for twin studies; i.e., they used systematic ascertainment, data collection, and diagnostic procedures, as well as blinding to co-twin diagnostic status (and/or zygosity). In addition, we limited our focus to categorical phenotypes for which heritability had been estimated in at least three separate samples. If more than one study reported on identical participants, we only included the most informative with regard to heritability. We calculated mean heritability estimates, weighted by the number of pairs in which at least one twin was affected, for the tables below. If the number of pairs with an affected twin was not reported, we calculated mean heritability estimates weighted by the total number of affected twins in each study. As used in this
