(AMY KING/THE WASHINGTON POST; IMAGES FROM ISTOCK)

Studies have long shown that women are more likely to suffer from depression and anxiety than men. And attempts have long been made to explain it, citing everything from biological differences to the challenges women disproportionately face, such as balancing the additional child care and family responsibilities often expected of them with their own careers.

But a new study by researchers at Columbia University's Mailman School of Public Health points to another possible culprit: The gender wage gap, and the potential underlying discrimination and biases that may go with it.

The study, published in this month's volume of the journal Social Science & Research, found that when a woman's income was lower than a male counterpart's, her odds of reporting anxiety disorder were more than four times higher than his. But if she made the same or more, her odds of suffering from it were much lower.

A similar pattern held true with depression. A woman who was paid less than her male counterpart had 2.4 times higher odds of depression; when her income was equal to or more than a male peer's, her odds of reporting multiple symptoms of depression were no different than his. Jonathan Platt, one of the authors of the paper and a doctoral student in epidemiology at Columbia, said in an interview that while his findings were in line with what they predicted, "I was surprised to see such clear differences. It was good to see the results, but also dismaying, of course, for what they represent."

Platt and his co-authors examined data on more than 22,000 working adults, putting them into matched pairs of men and women who were comparable in age and educational background, as well as who worked in similar industries, for similar types of employers and at similar occupation levels. While such matched pairs aren't perfect -- the counterparts didn't necessarily do the exact same job or work in the same organization -- they were a proxy that helped control for other possible explanations for the pay differences.

"One of the common questions about why women make less than men is that they're doing different kinds of work or have different life roles or work in environments that simply pay less," said Katherine Keyes, one of Platt's co-authors, in an interview. "It was really important if we were going to investigate the wage gap to be very diligent to ensure we could avoid that type of rebuttal." 

According to the U.S. Census Bureau, the median woman's earnings is 79 percent of the median income for men. Past studies have controlled for everything from education to job title to hours worked to years of experience and still found that some of the wage gap cannot be explained by these factors, and may be attributable to bias, whether explicit or not.

One natural question is whether lower incomes for women might simply be what's prompting higher odds of depression or anxiety. But Platt and Keyes said that while financial strain is a potential risk factor for the two disorders, the trend held true even among the men and women pairs who made plenty of money. "Even with executive-level occupations -- clearly with a high enough income to avoid material poverty -- we saw the relationship" between the wage gap and odds of mental health, Platt said.

Another question is whether it's the wage gap itself that could be upping the likelihood of depression and anxiety -- or whether it's actually discrimination or bias that underlies it. Keyes said they cannot be sure, as their research only measured the difference in wages itself. "But I would hypothesize that it's all the things that the wage gap represents," she said. "I think what makes this study really unique is the wage gap represents so much that is unseen."

After all, workplace bias often isn't something that's easily described or detected even by women themselves, much less studied by outside academics. The gap, Keyes said, is used "as a broad indicator of many things women appraise as discriminatory -- as well as things they don't."

Read also:

There has been no significant progress toward equal pay in seven years

The worst question you could ask women in a job interview

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