Stressful life events and affective illness

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.

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.

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.

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.