Minimal definitions of MDD do not simply include cases with lower genetic liability to MDD. This is consistent with a recent study of three large twin cohorts, which asked whether a combination of MDD, depressive symptoms and neuroticism could capture all genetic liability of MDD55 and showed that 65% of the genetic effects contributing to MDD are specific, and minimally defined depression (inclusive of MDD, depressive symptoms and neuroticism) can index only around one-third of the genetic liability to MDD. Similarly, previously reported high degrees of genetic correlation between MDD and depressive symptoms (rG = 0.7, implying that roughly rG2 = 49% of genetic factors contributing to liability of the former is attributable to that of the latter)22 need to be put in perspective of even higher degrees of sharing between depressive symptoms and other traits such as neuroticism (rG = 0.79–0.94, implying that roughly rG2=62–88% of genetic variance of the former is attributable to that of the latter, especially if both were assayed at a single time point56).
