Biological differences between the sexes may explain why rates of depression are higher in women, according to emerging research.
Scientists have long recognized that women have higher rates of depression, post-traumatic stress disorder and other anxiety disorders, but finding the reasons for this discrepancy has been challenging, said Debra Bangasser, an assistant professor of psychology at Temple University in Philadelphia.
"A lot of psychologists and clinicians have approached this question by looking at cognitive vulnerabilities, or if there are differences in reporting," Bangasser told LiveScience.
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"I think it's taken the neuroscience community a long time to change our thinking, to start looking for whether there are biological differences," she said. [Top 10 Controversial Psychiatric Disorders]
Animal studies
In a study published in January in the journal Biology of Sex Differences, Bangasser and her colleagues looked at the stress response systems in the brains of male and female rats. The researchers focused on receptors for two types of stress hormones: corticotrophin releasing factor (CRF) and glucocorticoids.
In the human brain, these receptors are activated in response to stress, and help people respond appropriately to the situation. For instance, if a driver hears squealing breaks, stress hormones and their receptors enable that person to look around for signs of danger while still watching the road.
When this response system is activated inappropriately, a person's stress response can become irregular, which, in mild cases can lead to anxiety, and in more serious cases can cause depression or post-traumatic stress disorder.
In their study, the researchers found that female rats were more sensitive to low levels of CRF, and were less adaptable to high levels of the hormone.
"You would expect CRF to activate the neurons in males and females equally, but the firing patterns in male rats look like our control animals without CRF," Bangasser said. This difference may mean that females more easily enter a state of hyperarousal, she said.
Further research is needed to see whether the same difference between sexes exists in people.
Rumination and depression
Biology may not be the only reason women are more prone to certain mental illnesses. Katie McLaughlin, an assistant professor of psychology at Boston Children's Hospital and Harvard Medical School, said that research shows how social and environmental factors might influence how people respond to stressful situations.
McLaughlin's work focuses on a process known as rumination, which occurs when people compulsively focus on the causes and consequences of their distress, rather than thinking about solutions to their problems. The idea of rumination was pioneered by Susan Nolen-Hoeksema, a professor of psychology at Yale University in New Haven, Conn., who died earlier this year.
Nolen-Hoeksema's research found that women are more prone to ruminate when they are depressed, whereas men were better able to distract themselves. While women may be biologically more prone to rumination, there may also be environmental factors that affect this, McLaughlin said.
"This might lead women to have a greater risk of depression," McLaughlin said at the annual meeting of the Association for Psychological Science in Washington, D.C., on May 25.
Identifying whether males and females are affected differently by depression will likely have important implications for depression treatments, particularly when drugs are involved, Bangasser said.
"Our understanding of sex differences and depression tells us that if these differences are biological, therapeutic drugs may work differently for males and females," she said. "The idea that there are basic sex differences is catching on, which I think is important, because majority of the disorders out there likely have a sex bias."
Follow Denise Chow on Twitter @denisechow. Follow LiveScience @livescience, Facebook & Google+. Original article on LiveScience.com.
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