Female interpersonal dependency: genetic and environmental components and its relationship to depression as a function of age

Objectives: Research has shown that female interpersonal dependency is significantly associated with clinical depression but is only marginally related to childhood environmental factors. This study replicates the finding of O'Neill and Kendler that such dependency has a significant genetic component and no shared environmental component and extends this by examining the effect of age and the relationship between interpersonal dependency and depression.Method: A genetic model analysis for female twin pairs was made incorporating a scale from the Interpersonal Dependency Inventory, and the association between dependency quartiles and depression for both sexes determined. Dependency data were obtained by questionnaire from 4427 monozygotic and dizygotic twins, 90% female and this was combined with lifetime incidence of depression data in three categories of severity.Results: Additive genetic variance components of 49% and 41% were estimated for those females between 19-64 and 65-87years, respectively, with no significant effect for the shared family environment for either age group. Only female dependency was found to be associated with the incidence of depression. Incidence levels of severe depression for older females in the highest quartile of dependency were 26% compared to 43% for the younger females.Conclusion: The investigation has estimated that nearly half of the variance in female interpersonal dependency is genetic in origin. It has also confirmed that high levels of such dependency are associated with the incidence of severe depression and this effect reduces with age. The possible reasons for this age-related effect are discussed.