Major depressive disorder is a chronic and persistent mental illness. EPIDEMIOLOGY Major depressive disorder (MDD) or unipolar depression is one of the most common psychiatric disorders seen in primary care, but accounts for only half of all major depressive disorder sufferers seek medical care (Johnson Vanderhoef 109; Myers DeWall 520). A LINK IS NECESSARY IN THE SENTENCE Prevalence of incidence. An estimated 5% of the United States population (or approximately 10,000,000 Americans) is affected by major depressive disorder, making it the leading cause of disability in the United States (Johnson and Vanderhoef 109). Unfortunately, 15% of major depressive disorder sufferers commit suicide (Johnson and Vanderhoef 109). Therefore, the importance of understanding the role of major depressive disorder in mental health is crucial. Women are more likely to suffer from major depressive disorder than men. In fact, there is “twice the prevalence of major depressive disorder in women compared to men” (Kaplan Sadock 528). According to Kaplan Sadock, the reason for this imbalance is due to: (a) hormonal imbalances, (b) childbirth, (c) various psychosocial stressors, and (d) behavioral patterns of learned helplessness (529). Age. Major Depressive Disorder can occur at any age, but the average age of onset is 40 (Kaplan Sadock 529). Johnson and Vanderhoef suggest that many MDD sufferers begin to have symptoms of MDD in their 20s and 30s (109). However, according to Kaplan Sadock, major depressive disorder can manifest itself both at an early age (childhood) and at an advanced age (529). Marital status. According to Kaplan Sadock, major depressive disorder occurs “most often in people without a close interpersonal relationship or in those who are divorced or separated” (529). This lack of a close personal relationship may be the trigger that exacerbates major depressive disorder. Kaplan Sadock suggests there is no correlation... half of article... Elphia, Pennsylvania: Lippincott Williams & Wilkins, 2012. Print."Depression (major depressive disorder)." Depression (major depression) Risk factors. Mayo Clinic, n.d. Web. May 27, 2014. Johnson and Kathryn, Dawn Vanderhoef. Psychiatric and mental health nurse review resource manual. 3rd ed. Silver Spring: American Nurse Credentialing Center, 2014. Print.Myers, David G., C. Nathan DeWall. Exploring psychology. New York, NY: Worth, 2014. Print. Stahl, Stephen. Guide for prescribers: Stahl's essential psychopharmacology. 4th ed. New York: Cambridge University Press. 2013. Print.Tusaie, Kathleen R., Joyce J. Fitzpatrick. Advanced practice psychiatric nursing: integration of psychotherapy, psychopharmacology, complementary alternative approaches. New York, NY: Springer, 2013. Print.