Affective Disorders: Oneormore

of affective disorders (bipolar affective disorder, major depressive disorder and anxiety states) result from multiple genes that confer a susceptibility or liability to develop the disorder when other (environmental) risk factors are also present. In general, bipolar affective disorder has been found to have the highest heritability (i.e. the proportion of variance explained by additive genetic factors) of around 80% (McGuffin et al, 2003), followed by major depression (between 40% and 70%, depending on the stringency of the definition; Kendler et al, 1993; McGuffin et al, 1996) and then by anxiety disorders, with heritabilities of around 40–50% (Eley et al, 2002). For affective disorders in adult life, the role as precipitants of certain proximal factors such as severe and threatening life events has been well replicated (Brown & Harris, 1978). There is also much evidence of distal factors such as childhood adversity contributing to vulnerability (Gilman et al, 2003). Important developmental aspects include the continuities between childhood depressive symptoms and adult depression and the changing contributions of genes and environment throughout the life span. For example, recent findings support and extend earlier work that has shown increasing genetic influence on depressive symptoms as children grow into adolescence (Scourfield et al, 2003).