Second, as discussed above, the genetic basis of most psychiatric disorders show fundamental connections. For example, the common variant genetic basis of MDD overlaps significantly with that of anxiety disorders, autism, ADHD, schizophrenia, bipolar disorder, smoking behavior, and anorexia nervosa (41). Moreover, the presence or absence of some clinical disorders (MDD, autism, and ADHD) shows strong genetic overlap with the analogous symptoms in general population samples. Further, the common genetic basis of many psychiatric disorders is often strongly correlated with that of putative subphenotyes (also known as endophenotypes or component phenotypes). For example, the common variant genetic basis of MDD is correlated with that for worse sleep, higher neuroticism, greater body mass in people without MDD (41). These results strongly suggest that our diagnostic categories do not define pathophysiological entities. The resolution of these issues will address major unanswered questions: from a genetic perspective, what are these disorders? How are they similar and how are they different?
