An analysis just published online has broken new ground by finding gender differences in both symptoms and diagnoses of depression appearing at age 12.

The analysis, based on existing studies that looked at more than 3.5 million people in more than 90 countries, confirmed that depression affects far more females than males.

The study, published by the journal Psychological Bulletin, should convince doubters that depression largely, but not entirely, affects females, says co-author Janet Hyde, a professor of psychology and gender and women's studies at the University of Wisconsin-Madison.

"We found that twice as many women as men were affected. Although this has been known for a couple of decades, it was based on evidence far less compelling than what we used in this meta-analysis. We want to stress that although twice as many women are affected, we don't want to stereotype this as a women's disorder. One-third of those affected are men."

The gender gap was evident in the earliest data studied by co-authors Hyde; Rachel Salk, now a postdoctoral fellow in psychiatry at the University of Pittsburgh School of Medicine; and Lyn Abramson, a professor of psychology at UW-Madison. "The gap was already present at age 12, which is earlier than previous studies have found," says Hyde. We used to think that the gender difference emerged at 13 to 15 years but the better data we examined has pushed that down to age 12."

The gender difference tapers off somewhat after adolescence, "which has never been identified, but the depression rate is still close to twice as high for women," Hyde says.

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Puberty, which occurs around age 12 in girls, could explain the onset, Hyde says. "Hormonal changes may have something to do with it, but it's also true that the social environment changes for girls at that age. As they develop in puberty, they face more sexual harassment, but we can't tell which of these might be responsible."

Although the data did not cover people younger than 12, "there are processes going on at 11 or 12 that are worth thinking about, and that matters in terms of intervening," Hyde says. "We need to start before age 12 if we want to prevent girls from sliding into depression. Depression is often quite treatable. People don't have to suffer and face increased risk for the many related health problems."

The results described averages across the nations covered in the study, Hyde says, but similar results emerged from the studies focusing on the United States.

The UW-Madison researchers looked at both diagnoses of major depression, and at symptom measure of depression, Hyde says. "Symptoms are based on self-reported measures -- for example, 'I feel blue most of the time' -- that do not necessarily meet the standard for a diagnosis of major depression. To meet the criteria for major depression, the condition must be evaluated much more rigorously."

The researchers looked at the relationship between depression and gender equity in income. Surprisingly, nations with greater gender equity had larger gender differences -- meaning women were disproportionately diagnosed with major depression. "This was something of the opposite of what was expected," says Hyde. "It may occur because, in more gender-equitable nations, women have more contact with men, and therefore compare themselves to men, who don't express feelings of depression because it doesn't fit with the masculine role."

Curiously, no relationship in either direction appeared for depression symptoms.

Despite the prevalence of and growing concern about depression, "this was the first meta-analysis on gender differences in depression," Hyde says. "For a long while, I wondered why nobody had done this, but once I got into it, I realized it's because there is too much data, and nobody had the courage to plow through it all. We did, and it took two years."