Psychologists and public-health experts are concerned with both the prevalence and the incidence of mental health problems. Prevalence refers to the frequency with which a given disorder occurs at a given time. If there were 100 cases of depression in a popu- lation of 1,000, the prevalence of depression would be 10%. The incidence of a disorder refers to the number of new cases that arise in a given period. If there were 10 new cases of depression in a population of 1,000 in a single year, the incidence would be 1% per year.
In 2005, the National Institute of Mental Health conducted a survey finding that 26.2% or approximately 57.7 million Americans were suffering from a mental disorder. While only about 6% were regarded as having a serious mental illness, almost half the peo- ple (45%) suffering from one mental disorder also met the criteria for 2 or more other mental disorders (Kessler, Chiu, Demler, & Walters, 2005). Notably, mental disorders are the leading cause of disability in the United States for people between the ages of 15 and 44 (The World Health Organization, 2004). Figure 12–1 shows the prevalence for some of the more common mental disorders among adult Americans. As shown in Figure 12–1, anxi- ety disorders are the most common mental disorder followed by mood disorders. (All of these are described in detail later in this chapter.)
More recently diagnostic interviews with more than 60,000 people in 14 countries around the world showed that over a 1-year period, the prevalence of moderate or serious psychological disorders varied widely from 12% of the population in the Americas to 7% in Europe, 6% in the Middle East and Africa, and just 4% in Asia (World Health Organization [WHO] World Mental Health Survey Consortium, 2004).
diathesis–stress model View that people biologically predisposed to a mental disorder (those with a certain diathesis) will tend to exhibit that disorder when particularly affected by stress.