As defined by the DSM-IV, BPD is highly heritable. There are substantial genetic and nonshared environmental correlations between mania and depression, but most of the genetic variance in liability to mania is specific to the manic syndrome.

Liability to DSM-IV major depression has a substantial heritable component, and there is no evidence of an effect of shared family environment. Some proband characteristics, especially shorter duration of episodes, may be associated with a larger degree of genetic determination.

Depressive disorders are more common in the relatives of depressed probands than in the population at large, and there is compelling evidence that the familial aggregation of bipolar disorder and severe unipolar depression is at least partly due to genetic factors. However, the evidence concerning 'non-endogenous' depression is less clear, and family environment probably plays a stronger role. Much current research is focused on two areas: firstly, the mode of inheritance of manic-depressive illness, with the use of molecular biological techniques to detect and localise major genes; and secondly, the ways in which familial predisposition and environmental insults combine to produce depressive disorder.

The relationship between life events and depressive disorder was assessed in 83 families ascertained through depressed probands. Contrary to expectation and to previous suggestions, we found no inverse relationship between the presence of familial loading and reactivity to stress. Thus the relatives of probands whose onset of depression followed life events or chronic difficulties had slightly higher lifetime rates of depression than the relatives of probands whose onset was not associated with adversity. There was only a weak and non-significant relationship between recent life events and current disorder among relatives, and no apparent tendency for life-event-associated depression to 'breed true' within families. Comparison with a community sample showed that the first-degree relatives of depressives had significantly elevated rates both of current depression and of recent threatening life events. This finding still held when proband-associated life events were discounted, suggesting that both liability to depression and propensity to experience life events are familial.