Genetic aetiology

There is evidence for the importance of genetics in the aeitology of severe unipolar depression:

• the risk of severe depression in the first degree relatives of a severely depressed patient is 10-15% (compared to 1-2% in the general population)

The evidence for a genetic aetiology of bipolar disorder is stronger:

• the concordance in twin studies is:
  • 70% for monozygotic twins reared together
  • 70% for monozygotic twins reared apart
  • 23% for dizygotic twins
• in adoption studies, risk for bipolar affective disorder stems more from the genetic rather than the adoptive parent

The inheritance is probably non-Mendelian.