Nearly one in five Americans, or 45 million adults, experienced some form of mental illness last year, according to a major US government survey published on Friday.
The 18-25 age group reported the most mental illness, and more women than men were afflicted, said Peter Delany, a doctor who heads behavioral research at the Substance Abuse and Mental Health Services Administration.
"One in five people have suffered from a mental illness in the past year. That is a lot of people," Delany told AFP.
About one in 20 individuals, or around 4.8 percent of the population, met the criteria for having a serious mental illness.
The study defined mental illness as "a diagnosable mental, behavioral, or emotional disorder in the past year, regardless of their level of functional impairment."
A vast number of conditions could fall under that category, including anorexia, bulimia, bipolar disorder, kleptomania, depression, obsessive compulsive disorder, voyeurism, pathological gambling or insomnia.
Some of the same disorders could also land a person onto the "severe mental illness" side if he or she experienced the effects to such a degree that they caused a major disruption to daily life.
Mental illness was classified as severe when it "resulted in serious functional impairment, which substantially interferes with or limits one or more major life activities."
Severe mental illness would typically interrupt a person's life for at least two weeks and could include bipolar episodes, psychosis and attempted suicide as well as more extreme mood or eating disorders.
Delany described as "significant" the findings that a full 30 percent of the 18-25 age group reported suffering some form of mental illness in the survey, which was carried out in 2009.
"These are the people that are going to college. These are the people that are going into the workforce. These are the individuals that are getting married and starting families," said Delany.
"In many cases, all of these groups aren't really getting significant help."
Women were more likely to have experienced mental illness, with 23.8 percent of women reporting such disorders compared to 15.6 percent of men.
Delany said the survey did not go into the causes of mental illness, but that the male-female findings were consistent with the bulk of clinical research today.
The annual survey is the largest of its kind undertaken by the US government.
Researchers obtain the results by visiting residences door-to-door and handing people a small audio-CASI computer and headphone set with which they can privately answer the survey.
Delany said the research compiled responses from 68,000 people aged 12 and older who were not living in institutions, jails or on the street, and were not members of the US military.
Explore further: Could summer camp be the key to world peace?