Clinical depression affects about 7% - 18% of the population on at least one occasion in their lives, before the age of 40. In some countries, such as Australia, one in four women and one in six men will suffer from depression. In Canada, major depression affects approximately 1.35 million people [6]. The mean age of onset, from a number of studies, is in the late 20s.[citation needed]. Because people who have one episode of depression may have more in the future, the first time a young person becomes depressed is important both as a personal and public health concern [7].

About twice as many females as males report or receive treatment for clinical depression, due to stress and adversity, though this imbalance is shrinking over the course of recent history; this difference seems to completely disappear after the age of 50–55. Clinical depression is currently the leading cause of disability in North America as well as other countries, and is expected to become the second leading cause of disability worldwide (after heart disease) by the year 2020, according to the World Health Organization.[8]

According to recent studies , the diagnostic criteria for depression is far too broad, leading to people who are not truly clinically depressed being diagnosed due to a normal reaction to negative events.