Two sources of genetic liability to depression: Interpreting the relationship between stress sensitivity and depression under a multifactorial polygenic model

Psychopathology theories, clinical observations, and research all point to multiple sources of liability to depression. This article uses a longitudinal twin-study design to characterize the contribution of two geneticallyinfluenced sources of depression risk: The first corresponding to stress sensitivity and the second representing risk that is independent of stress sensitivity. The sample consisted of 606 pairs of same-sex adolescent twins recruited from Beijing, China. Mean (SD) age at intake (Wave1) and follow-up (Wave2) was 13.2 (2.6) and 15.1 (2.6) years, respectively. A Reaction Level index was developed to reflect individual differences in stress sensitivity. Biometric models were fit to examine the genetic influence on the variance of and covariance between stress sensitivity and depressive symptoms. Results showed that both Reaction Level and depressive symptoms were moderately heritable. The genetic correlation between depressive symptoms and Reaction Level was estimated to be .884. Genetic contributions to Reaction Level accounted for 37.5% of the total variance of depressive symptoms. Another set of genetic factors, which did not contribute to Reaction Level, accounted for 10.5% of the total variance of depressive symptoms. We interpret our results within the context of a multifactorial polygenic model, whereby depression risk is due to the combined contribution of multiple genetic and environmental factors.