Epidemiology

Depression is a major cause of morbidity worldwide. Lifetime prevalence varies widely, from 3% in Japan to 17% in the US. In most countries the number of people who would suffer from depression during their lives falls within an 8–12% range. In North America the probability of having a major depressive episode within a year-long period is 3–5% for males and 8–10% for females.^[204][205] Population studies have consistently shown major depression to be about twice as common in women as in men, although it is unclear why this is so, and whether factors unaccounted for are contributing to this. The relative increase in occurrence is related to pubertal development rather than chronological age, reaches adult ratios between the ages of 15 and 18, and appears associated with psychosocial more than hormonal factors.

People are most likely to suffer their first depressive episode between the ages of 30 and 40, and there is a second, smaller peak of incidence between ages 50 and 60. The risk of major depression is increased with neurological conditions such as stroke, Parkinson's disease, or multiple sclerosis and during the first year after childbirth. It is also more common after cardiovascular illnesses, and is related more to a poor outcome than to a better one.Studies conflict on the prevalence of depression in the elderly, but most data suggest there is a reduction in this age group.

Depression is often associated with unemployment and poverty. Major depression is currently the leading cause of disease burden in North America and other high-income countries, and the fourth-leading cause worldwide. In the year 2030, it is predicted to be the second-leading cause of disease burden worldwide after HIV, according to the World Health Organization. Delay or failure in seeking treatment after relapse, and the failure of health professionals to provide treatment, are two barriers to reducing disability.

The World Health Organization updated its report The global burden of disease in 2004. Their "Years Lost due to Disability", or YLD, is a measurement of the equivalent years of healthy life lost through time spent in states of less than full health, and they state that in all regions, "neuropsychiatric conditions are the most important causes of disability, accounting for around one third of YLD among adults aged 15 and over." Specifically, unipolar depressive disorders are the leading cause in both males and females, in high-income countries and in low- and middle-income countries.