To be sure, not every woman is a chronic worrywart. But women are twice as likely as men to be diagnosed with anxiety disorders, according to the National Institute of Mental Health.

Understanding why isn't easy. In the last two decades, researchers have examined hormonal fluctuations, genetics, environmental stressors and cultural factors. And they've concluded there's no single reason women are more vulnerable to anxiety than men.

It's really the interplay between all these factors that leads to higher anxiety rates in women compared to men," said Olga Brawman-Mintzer, director of the anxiety disorders program at the Medical University of South Carolina in Charleston.

The brain is chock full of receptors for female hormones. These hormone receptors also interact with brain chemicals called neurotransmitters, which are involved in feelings of anxiety. These interactions may be part of the reason why women are at higher risk for anxiety than men. A 2006 review article in the journal Neuropsychopharmacology found that women's risk for anxiety (and its close cousin, depression) goes up after puberty, when estrogen production begins to rise. But the estrogen-anxiety relationship isn't linear, Brawman-Mintzer warns.

"It's not a very simple relationship," she said. "At low levels, estrogen may have anti-anxiety effects, but very high levels may have negative effects."

Some of the gender difference could be evolutionary. Research has shown that both adult women and young girls are more likely than men and boys to make connections between bad events in the past and possible negative events in the future, which could also increase anxiety about what lies ahead. It's possible that these coping mechanisms may have helped our ancestors successfully raise offspring, but in today's world, they might make women more vulnerable to worry.

And then there are environmental factors. Societal expectations can shape anxiety symptoms, and women may face different cultural burdens than men. In one study, researchers asked Swedish women about their health and stress levels, and found that a combination of job strain and unpaid housework lead to poorer self-reported health, according to an article in the International Journal of Behavioral Medicine in 2006.

Whatever the cause, anxiety is a serious problem. Chronic stress can weaken the immune system and has been linked to heart disease, Type 2 diabetes and some types of cancer. And, according to 2008 data from the National Institute of Mental Health, more than 40 million American adults have a full-blown anxiety disorder. So take a deep breath, relax, and if that doesn't work, see a doctor.