Mental Illness in the USA

I have been consistently surprised by the numbers of people in the USA diagnosed with a mental illness. For example, consider this statement from an excellent review on the apparent epidemic of mental illness in the USA:

The number of people treated for depression tripled in the following ten years, and about 10 percent of Americans over age six now take antidepressants. The increased use of drugs to treat psychosis is even more dramatic. The new generation of antipsychotics, such as Risperdal, Zyprexa, and Seroquel, has replaced cholesterol-lowering agents as the top-selling class of drugs in the US. 

The Epidemic of Mental Illness: Why?, Marcia Angell, The New York Review of Books