Stressful life events and affective illness

Objective: To investigate the association between the occurrence of affective illnesses and the number/type of experienced negative stressful life events in a twin material. A case-control study with an unrelated twin as control to the case and a co-twin control study were both undertaken with the same material. Method: Postal questionnaire responses were used for confirming diagnosis and to inventory stressful life events. Risk ratios (RR) for groups of events were calculated using conditional logistic regression. Results: A dose-response relationship was observed for the association between stressful life events and affective illness. The RRs for specific groups of exposures were higher in the co-twin control study and higher for dizygotic twin pairs than for monozygotic twin pairs. Conclusion: Individuals with a history of affective illness select themselves into high-risk environments, in part due to their genetic propensity to the disease. Thus, the association represents a classic genotype-environment correlation.