The number of adults in the United States who suffer from major depressive episodes at some point in their life is far higher than previously believed, a new study by the Yale School of Public Health finds.

National survey data currently shows that approximately 17% of women and 10% of men report having a history of major depressive episodes (MDEs) in their lifetimes. But these data are subject to "recall error," or the tendency of people to forget or misreport their health histories when taking a survey.

Researchers led by Jamie Tam, Ph.D., assistant professor in the Department of Health Policy and Management, created a simulation model to generate corrected estimates of lifetime depression. They found that the proportion of U.S. adults who have had MDEs is actually closer to 30% of women and 17% of men after factoring in recall error.

"Major depressive episodes are far more common than we thought," said Tam. "Our model shows that the probability of someone having a first major depressive episode is especially high during adolescence. We also know from other research that having a first major depressive episode increases the likelihood you'll have a second one. This means that anything we can do to prevent or treat episodes among young people could lead to larger health benefits over the course of their life."

The findings are published in the American Journal of Preventive Medicine.

A major depressive episode is defined as a period of two weeks or longer in which a person experiences feelings of intense sadness and hopelessness, fatigue, weight gain or weight loss, changes in sleeping habits, loss of interest in activities and thoughts of suicide or attempts at suicide. These persistent symptoms cannot be easily changed, even if they are contradictory to a person's circumstances. Depressive episodes typically recur periodically in people diagnosed with major depression.

The study shows that mental health programs that screen for, prevent and treat depression could benefit a much larger segment of the population than previously thought, Tam said.

"If you think about chronic health conditions like heart disease, we do a lot to identify people who might be at risk for additional health events like heart attacks because that group would benefit from maintenance treatment and clinical monitoring," Tam said. "We don't do such a great job when it comes to mental health conditions. So, if we're able to assess how many people actually have histories of depression, that also tells us that more people are at risk of experiencing more depressive episodes."

The researchers also found that older adults are especially likely to under-report their history of having depressive symptoms. Among adults 65 years and older, underreporting for depression was as high as 70%. Older adults often experience what is referred to as "minor depression," where they still report significant depressive symptoms but don't always meet clinical requirements for major depression. Tam said there may be a tendency for older adults to downplay negative experiences of depression from when they were younger, classifying them as "growing pains" rather than major depression.

"Unfortunately, many people with depression or with histories of depression don't access, or don't have access to, treatment or support," Tam said. "There's a broader problem in our society of mental health not receiving the same attention and investment of resources compared to physical health conditions."