Mental illness is an umbrella term denoting any one or more of the mental disorders listed in DSM-IV or ICD-10.3 The hallmarks of these disorders are abnormalities in mood, cognition, and the highest integrative aspects of human behavior, such as planning and social interactions. Mental illness is highly prevalent, with about 20 percent of the United States population (about 44 million) fulfilling criteria for one or more disorders in a given year (DHHS, 1999). Anxiety (16 percent) and depression (6–7 percent) are the most common, whereas bipolar disorder and schizophrenia affect about 1–2 percent of the population.

While overall rates of mental illness do not vary by gender, women have significantly higher rates of major depression: the 12-month rates are 13 percent of women versus 8 percent of men (Kessler et al., 1994). About half of people with a mental illness will also have a substance use disorder at some time during their lifetime (Kessler et al., 1994; DHHS, 1999). Comorbidity of MI and substance use disorders is the norm, rather than the exception.

People within the lowest socioeconomic status (SES) group are about two to three times more likely to suffer from mental illness than people in the highest group. Minorities, by virtue of lower SES, are disproportionately affected by mental illness (DHHS, 2001). After controlling for SES, mental illness is as prevalent in African Americans and Hispanic Americans as whites. The two main explanations for the link between poverty and mental illness are that (1) poverty causes exposure to more stressful environments (with fewer social supports), and (2) poverty is a consequence of having a mental illness that leads to unemployment or underemployment (DHHS, 2001). The fact that MI is more prevalent in minority populations is critical to understanding the shifting dynamics of the HIV epidemic, as explained later.

Mental illness is highly disabling, especially depression, bipolar disorder, or schizophrenia. A groundbreaking study by the World Health Association (WHO) ranked mental illness first in terms of causing disability in the United States, Canada, and Western Europe (WHO, 2001). It found that mental illness accounts for 25 percent of total disability, a rate higher than that for substance use disorders, which ranked second.

The disability toll of mental illness is high because mental illness is highly prevalent, often arises in childhood or adolescence, and carries a long-term (usually relapsing–remitting) course. Mortality is more commonly


Diagnostic and Statistical Manual of Mental Disorders, DSM-IV, 4th edition. Published by the American Psychiatric Association, 1994; International Classification of Diseases, 10th edition. Published by the World Health Organization.

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