Mood instability underlies the relationship between impulsivity and internalizing psychopathology.

Impulsivity, the tendency to act without adequate forethought, has been associated with various internalizing disorders. Mood instability, the tendency to experience rapid and intense mood swings, relates to both internalizing disorders and impulsivity. We hypothesized that the association between mood instability and impulsivity accounts for the relationship between impulsivity and internalizing psychopathology. We conducted two studies to test our hypothesis. In Study 1 we used data from the 2007 Adult Psychiatric Morbidity Survey to examine mood instability in the relationship between depression and impulsivity. Mood instability and impulsivity were assessed with the Structured Clinical Interview for DSM-IV Axis-II Personality Disorders and depression was assessed with the revised Clinical Interview Schedule. In Study 2 we used data from the 1984 and 1991 British Health and Lifestyle Surveys to examine mood instability in the longitudinal relationship between impulsivity and internalizing symptoms. Mood instability and impulsivity were measured with the Eysenck Personality Inventory and internalizing symptoms were assessed with the General Health Questionnaire. In both studies we used a sequential regression analysis to test our hypothesis. Results from Study 1 showed that participants with depression were more likely to report impulsivity, but this effect became nonsignificant when mood instability was included in the same regression model. In Study 2 impulsivity predicted internalizing symptoms seven years in the future, but this effect became nonsignificant after mood instability was included in the same regression model. We conclude that impulsivity relates to internalizing psychopathology largely by being associated with mood instability. Research and therapy for internalizing conditions might be more productively directed at mood instability rather than impulsivity.