Nearly one in 10 Americans is depressed, and one in 30 meets the criteria for major depression, with the rate higher among the unemployed or those who can't work, a study said Thursday.
Nine percent of more than 235,000 adults polled from 2006-2008 in 45 US states, the capital Washington, Puerto Rico and the US Virgin Islands, met the criteria for depression, and 3.4 percent for "major" depression, according to the study by US Centers for Disease Control and Prevention (CDC).
Among people who classified themselves as unable to work on the survey analyzed by the CDC for the report, nearly a quarter -- 22 percent -- met the criteria for major depression, as did nearly 10 percent of those who said they were unemployed.
Although the survey did not ask respondents why they were unable to work, McKnight-Eily said they were probably disabled or suffering from long-term illness.
In sharp contrast to the unemployed, only two percent of people with jobs had symptoms of major depression, the study published in the CDC's weekly Morbidity and Mortality report (MMWR) said.
Participants were deemed to be suffering from major depression if they met five of eight criteria on a questionnaire that asked how often during the previous two weeks they had feelings of hopelessness or disinterest, if they had trouble falling asleep or if they slept too much.
The questions also inquired about respondents' appetite, concentration, restlessness, lethary or feelings of failure.
The real rate of depression among adult Americans was likely to be significantly higher because key groups -- the homeless and the incarcerated -- were not included in the survey, study co-author and clinical psychologist Lela McKnight-Eily told AFP.
The survey also found that just under seven percent of people who had not completed high school suffered major depression compared to four percent of high school graduates and 2.5 percent of people who had spent at least some time at university.
Women were more susceptible to depression than men, young people were more likely than those over 65 years old to suffer from depression, and blacks and Hispanics were more likely than whites to be depressed, it said.
Depressive disorders are also "more common among persons with chronic conditions," such as obesity, cardiovascular disease, diabetes, asthma, arthritis and cancer, the study said.
Sometimes, chronic illness and depression work together to negatively impact the sufferer, said McKnight-Eily.
"A person who has a chronic disease and becomes depressed may stop following the directions on their prescription medications and their condition could get worse, which might make them even more depressed," she added.
A separate short report in the MMWR said depression can result in "increased work absenteeism, short-term disability and decreased productivity."
Depression was the third leading cause of disease burden worldwide in 2004 and is expected to be second only to cardiovascular disease by 2020, the CDC said in the twin reports, issued a week before US National Depression Screening Day on October 7.
Explore further: What makes a good horror movie?