Mental illnesses are both prevalent and undertreated in many countries, particularly in the United States, according to a study from the World Health Organization. Rates vary greatly by location. For example, about 5% of people living in Nigeria have symptoms of mental disorders, compared with 26% of people in the United States. However, researchers said that language barriers and societal variations affecting a person's willingness to disclose personal information about mental illness may explain some of these differences.

Researchers conducted face-to-face interviews in the homes of 60,463 adults living in 14 countries to assess for a wide range of mental illnesses. The most common type found in all countries (except the Ukraine) were anxiety disorders, such as panic attacks, phobias, and posttraumatic stress disorder.

In the United States, between 35% and 50% of people with serious mental illness hadn't received treatment during the year prior to the interview.

In all countries, researchers found that a larger portion of people with less severe mental disorders had received treatment, compared with those with more serious disorders. For example, more wealthy suburban Americans are treated for mild mental illnesses than poor Americans are for serious illnesses. The researchers conclude that their findings suggest a misallocation of treatment resources.

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