Social factors and comorbidity of depressive and anxiety disorders.

Differences in rates of depression across and between populations can be considerable and are largely determined by psychosocial factors. Such findings have important implications for comorbidity. On the basis of multiplying independent probabilities, the proportion of comorbid conditions increases as base rates of the disorders increase in a population. If such a chance combination has clinical implications, it would appear to be a "fact" of significance irrespective of biological underpinnings. In a recent survey of 404 women living with at least one child in an inner-city area, the rate of both anxiety and depression was highly related to the childhood experiences of neglect and abuse. However, adversity in adult life (e.g. widowhood or divorce), which might be expected to relate to current stressors, was only related to the rate of depression. The two risk factors had a considerable impact on comorbidity by increasing the rate of each disorder and thus the probability of their occurring together. They had a lesser impact as "common antecedents". If both influences are considered, such adversity explains around half of the comorbidity. This is a conservative estimate of the impact of psychosocial factors.