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The American novelist Nathaniel Hawthorne once likened happiness to a butterfly fluttering just beyond grasp. No one will debate that happiness is elusive, sometimes proving difficult to find, and for some, completely out of reach. According to medical and economic research, there is a significant gender imbalance in the experience of that helpless blue feeling. Women are more likely than men to report unhappiness and suffer clinical depression.

Ten to 14 million Americans experience a clinical depression each year, and women are twice as likely to be the sufferers. The odds a woman has been diagnosed with major depressive disorder in the past year are 1 in 11.63, while the odds for a man are 1 in 20.41. Similarly, women suffer from higher rates of seasonal affective disorder and chronic depression. Women are more than twice as likely as men to report taking antidepressants. Though bipolar disorder affects men and women equally, women spend more time in the depressed phase.

Researchers suspect hormones. Phases unique to the female body, such as menstruation, pregnancy, childbirth, and menopause, coincide with occurrences of female depression. Mood and behavioral changes occur in 20%-40% percent of premenstrual women. Ten to 17% of women experience depression during pregnancy. In the postpartum period, over 80% of women experience minor mood changes, but 25% of those who report these blues go on to suffer severe postpartum depression. It makes sense. Constantly fluctuating estrogen and progesterone levels throughout the female reproductive cycle have major effects on brain systems that govern mood. Researchers are beginning to assume that the hormone deluges of female fertility increase the susceptibility to stress and risk for depression.

But attributing women’s unhappiness to fluctuating hormones may be an easy out—it ignores life situations such as societal status, individual responsibilities, single parenthood, professional success, and the like. That’s where a large-scale 2009 study by two economists (a man and a woman, Justin Wolfers and Betsey Stevenson, of the Wharton School at the University of Pennsylvania) comes in. Published in the American Economic Journal: Economic Policy, the study found that over the past 35 years, women’s subjective reports of happiness have declined relative to male happiness even though opportunities for women have increased. And the easy explanations don’t suffice—the trend occurred whether the women and men were black or white, married or divorced, parents or nonparents, educated or uneducated.

The researchers struggle with endless possibilities. Women could be answering the survey more honestly than before. They could be tougher on themselves than past generations. Men could be engaging in more enjoyable activities. But though the researchers declare that their goal is only to document the trend, not to explain it, they do have one telling piece of data. Describing the parallels between areas that show a decline in happiness for both men and women, they write: “The one clear exception is that women have become less satisfied with their family’s financial situation both absolutely and relative to that of men.” Breadwinners more than ever before, women are concerned for the well-being of their families and the prospects for their children.