There is no known single cause of depression; rather depression is generally attributed to a combination of psychological, social and biological factors. Numerous factors affect a person’s likelihood of developing depression, including: stressful life events, their age, sex, socioeconomic and marital status, hereditary factors, childhood adversities as well as mental and physical comorbidity.

Prevalence highest in adulthood

Depression rarely occurs before puberty; prevalence rates start rising from about age 12. Prevalence rates increase continuously until the age 60 when they start declining (Ernst & Angst, 1995). There are very few epidemiological surveys assessing depression morbidity in the very old. Whether depression prevalence decreases or only depression expression changes continues to be debated (Ernst & Angst, 1995; Henderson et al., 1998); (Stordal et al., 2003; Roberts et al., 1997). In old and very old age there is a shift in the presentation of depressive syndromes away from the classic picture of major depression towards a more sub-clinical and somatic manifestation (Ernst & Angst, 1995, Bramesfeld et al., 2007a).

Depression more common in women than in men

Depression is more common in women than in men. Consistently epidemiologic surveys have found prevalence rates of depression to be twice as high in females. The reasons for that are not understood. Hormonal and genetic factors as well as gender differences in living circumstances (such as poverty, single parenthood, dependent life style) and coping styles (internalisation, a stronger interpersonal orientation) are discussed in this context. Whereas attempted suicide rates are higher for females in most countries the opposite is true for completed suicides (Värnik et al., 2009).

Social factors: marital status and socioeconomic status

Widowed and divorced persons have a higher risk of developing depression (Klose & Jacobi, 2004). This is also true for parents who are single or unemployed and have dependent children (Helbig et al., 2006). There is a clear socioeconomic gradient in depression risk. The higher the educational degree and the higher the social status (as a construct made of education, work status and income), the lower the depression risk is. Unemployment is associated with a doubled risk for depression (Fryers et al., 2005). All these studies, however, do not allow to draw strong conclusions about causality because especially recurrent or chronic depression often causes socioeconomic decline, divorce and unemployment.

Also see Predictors of mental disorders

Childhood adversities are associated with depression

Adversity is a major negative event in childhood or adolescence challenging seriously one's ability to cope. Such adversities are sexual abuse (rape and sexual molestation), physical abuse and serious neglect before the age of 18. Childhood adversities are associated with depression and its prognosis in adulthood, as well as with increased somatic morbidity and mortality in both childhood and adulthood. Childhood adversities are strongly associated with vulnerability and several background factors (such as personality and genotype) moderate the outcome of the adversity (MINDFUL, 2008).

Substantial evidence supports the heritability of lifetime major depression

People who have relatives who have had clinical depression have a greater chance of developing it themselves. The heritability of lifetime major depression as assessed at personal interviews in over 15,000 complete pairs of twins from the national Swedish Twin Registry, was estimated to be 38% (Kendler et al., 2006). Similar result of 37% heritability of major depression was obtained from a meta-analysis of other twin studies (Sullivan et al., 2000). There was a difference with regards to gender; the heritability of major depression was estimated at 42% in women and 29% in men (Kendler et al., 2006). There is no single major gene responsible for depression. However, a larger set of genetic variants showing complex interactions among themselves as well as with environmental factors are supposed to influence the risk of a certain person to become depressed.