Background. Although prior research has demonstrated a strong association between interpersonal dependency (IPD) levels and risk for major depression (MD), the possible aetiological explanations of this association as well as any gender differences in the IPD–MD relationship need further clarification.

Method. Population-based twin samples (N=7174) were interviewed in multiple waves to assess IPD and MD as part of a larger twin study. IPD levels were assessed using the Interpersonal Dependency Inventory while MD diagnoses were derived from the SCID. Cox proportional hazard models and multiple regression techniques were utilized.

Results. IPD was strongly associated with a risk for lifetime MD. Pre-morbid IPD scores were predictive of future onsets of MD while experiencing a MD episode was also associated with a significant rise in IPD levels. While females had higher IPD scores, IPD scores were more significantly associated with risk for lifetime MD in males. Controlling for the level of IPD substantially reduced the observed association between gender and risk for MD.

Conclusion. The strong association observed between IPD and risk for MD results largely from IPD being a risk factor for MD, but state effects of MD on IPD also contribute. IPD scores in males were more predictive of lifetime MD than for females. The higher levels of IPD in women than in men may contribute meaningfully to the sex differences in risk for MD.