Researchers are constructing scientific theories to explain why women are nearly twice as likely as men to become depressed. Social bias and women’s higher rates of physical and sexual abuse and poverty, experts say, clearly play a role. But scientists are also studying genes that may predispose girls and women to the disorder.

They are examining the likely role of estrogen and even linking the development of clinical depression to negative thinking, which is more common in women than in men.

There is no question that women bear the brunt of the illness that Winston Churchill referred to as his “black dog.” The National Comorbidity Study, a large survey of adults in the United States released last year, found that 1.7 women for every man had experienced at least one episode of depression. Roughly the same ratio has been found in recent studies in nine other countries, including Canada, Brazil, Germany and Japan, said Dr. Marta Meana, an associate professor of psychology at the University of Nevada at Las Vegas.

“This is a global phenomenon,” said Dr. Meana.

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