Based on results from their study of 1765 twins, Kendler et al1 concluded that the familial and genetic features of major depressive disorder (MDD) were characterized by recurrence, long duration of episode, impairment, and suicidal thoughts or completions. They concluded, in contradiction to at least 6 independent family studies, that early age of onset (<40 years) is probably not familial or genetic. Based on their twin study findings, they also suggested that the National Institute of Mental Health (NIMH) Genetics Workgroup's conclusion and subsequent NIMH proposal for sibling pair collection of probands with early-onset recurrent MDD were unjustified.
Weissman MM, Wickramaratne P. Age of Onset and Familial Risk in Major Depression. Arch Gen Psychiatry. 2000;57(5):511. doi: