The Heritability of Somatic Symptoms in a Sri Lankan Twin Sample

Few twin studies have examined large representative samples in nonwestern, developing countries. This is important to address because the relative magnitude of environmental influences on common mental disorders may differ in populations experiencing qualitatively and quantitatively different environmental risk factors to those commonly experienced in more developed countries. It has been suggested that depressed people in nonwestern populations often present with a high rate of somatic symptoms. We measured somatic symptoms that are cross-nationally associated with depression and anxiety, using the Bradford Somatic Inventory (BSI), on our population-based twin sample (N = 930 adult pairs). We ran variance components models in Mx to assess the relative contribution of genetic (A), shared environmental (C) and nonshared environmental (E) factors to the variance in BSI scores.

The shared environmental factor accounted for 20% of the variance in BSI scores for females, but none for males. The rest of the variance was accounted for by genetic and nonshared environmental factors in approximately equal proportions. Environmental as well as genetic factors are important in explaining somatic symptoms in Sri Lanka, and the environmental influences differ in men and women. These results are a good indicator of the relative importance of genetic and environmental factors on anxious and depressive disorders in this population.