Background: A bias to develop negative affect in response to daily life stressors may be an important depression endophenotype, but remains difficult to assess.
Aims: To assess this mood bias endophenotype, uncontaminated by current mood, in the course of daily life.
Method: The experience sampling method was used to collect multiple appraisals of daily life event-related stress and negative affect in 279 female twin pairs. Cross-twin, cross-trait associations between dailylife mood bias and DSM-IV depression were conducted.
Results: Probands whose co-twins were diagnosed with lifetime depression showed a stronger mood bias to stress than those with co-twins without such a diagnosis, independent of probands' current depressive symptoms and to a greater extent in monozygotic twins than in dizygotic twins.
Conclusions: Genetic liability to depression is in part expressed as the tendency to display negative affect in response to minor stressors in daily life. This trait may represent a true depression endophenotype.