ABSTRACT
Major depressive disorder (MDD) is among the most prevalent of all psychiatric disorders. Recent estimates indicate that almost 20% of the American population, or more than 30 million adults, will experience a clinically signi˜cant episode of depression during their lifetime (Kessler and Wang 2009). Moreover, depression is frequently comorbid with other mental and physical dif˜culties, including anxiety disorders, cardiac problems, and smoking (e.g., Freedland and Carney 2009). Depression also has signi˜cant economic and social costs. Kessler et al. (2006), for example, estimated that the annual salary-equivalent costs of depression-related lost productivity in the United States exceed $36 billion. Given the high prevalence, comorbidity, and costs of depression, it is not surprising that the World Health Organization Global Burden of Disease Study ranked this disorder as the single most burdensome disease worldwide (Murray and Lopez 1996). Finally, it is important to note that depression is a highly recurrent disorder. More than 75% of depressed patients have more than one depressive episode, often relapsing within 2 years of recovery from a depressive episode (Boland and Keller 2009). Indeed, between one-half and two-thirds of people who have ever been clinically depressed will be in an episode in any given year over the remainder of their lives (Kessler and Wang 2009).
