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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.

There are several methodologic issues that need to be considered before their findings on age of onset can be accepted. First, the mean ± SD age of onset in their twins is 25.9 ± 10.2 years. This implies that most onsets were at younger than 40 years, which is the age of . . . [Full Text of this Article]