Considering the prevalence of domestic violence, it's easy to assume that intimate partner violence (IPV) is the source of depression and suicidal behavior in women. New evidence, however, is showing that the relationship between IPV and depression is a more complicated one.A study conducted by Karen Devries from the London School of Hygiene & Tropical Medicine along with colleagues found that symptoms of depression, among women, were associated with incident experiences of violence.

The relationship between violence and depression, then, may very well work in both directions.

Published today in PLOS Medicine, Devries' study is not the first to summarize the impact on mental health from various forms of violence and abuse against women. What is novel to her project, however, is its focus on longitudinal studies, which are conducted over long periods of time. Given this, they are able to examine the chronological relationship between intimate partner violence and depression.

The authors searched more than 22,000 records from 20 databases for studies examining physical and/or sexual intimate partner or dating violence and symptoms of depression, diagnosed major depressive disorder, dysthymia, mild depression, or suicide attempts. Sixteen studies with 36,163 participants met their criteria. Although all studies included women participants, four also included men.

Clear evidence of an association between IPV and incident depressive symptoms existed among the women; 12 of 13 studies showed a positive direction of association and 11 reached statistical significance. IPV was also associated with incident suicide attempts.

Yet an unexpected chicken-or-the-egg argument of origin was hatched. In their examination of longitudinal studies, the researchers found evidence of an association in the reverse direction - depressed women somehow encouraged their own domestic abuse.

For men, evidence suggested IPV was associated with incident depressive symptoms, but no clear evidence could be found associating IPV with suicide attempts or, for that matter, associating depression with incident IPV.

In his examination of Devries' work, Alfred Tsai suggested that another potential variable, unmeasured in the study, might be part of the mix. A potential history of childhood abuse might be causally related to both adult depression and adult abuse, irrespective of their relationship with each other. Tsai argued that by not taking childhood abuse into account, a "spurious association" between depression and IPV could mistakenly be interpreted as bidirectional.

According to World Health Organization, in eight population-based surveys from around the world, 10 to 69 percent of women reported being physically assaulted by an intimate male partner at some point in their lives. In the U.S., 22 percent of women over the course of their lifetime report being physically assaulted by an intimate partner; in South Africa, that number is 13 percent. Globally, more than 350 million people of all ages suffer from depression.

Depression is the leading cause of disability worldwide and is a major contributor to the global burden of disease.

Although Tsai supports the suggestion by Devries and colleagues that violence against women may have direct adverse effects on mental health, he asserts that both violence and depression may be rooted, partly, in gender-unequal relations and the "unremitting strain of poverty."

For example, in one randomized controlled trial conducted in rural South Africa, partnered women who took part in a microfinance intervention experienced a statistically significant decline in intimate partner violence. An analysis of macroeconomic data showed that changes favoring women in labor market condition in the U.S. led to reductions in domestic violence.

Improvements in women's bargaining power vis-a-vis their intimate partners, although an individual and microscale change, might be part of an overall policy intervention targeting the conditions that shape women's risks for both IPV and depression.