Environmental Influences

There are a lot of studies, which associate environmental influences in the onset of depression. Various studies even showed that familial conflicts, relationship problems, and low social class are associated with the early onset of depression (Fu & Parahoo 2009); while, positive events or absences from stresses caused a decrease in the chance of developing depression (Friis et al. 2002).

Figure 3. Young adult depression is mainly caused by life events during childhood.

Role of Genetics

Though environmental factors are associated with the onset of depression, various family, adoption and twin studies show that genetic factors can also cause depression. Some twin studies even confirmed that the genetic risk for developing clinical depression is 40% (Black Dog Institute 2008). In addition, a family study of early-onset depression showed that almost one-third of first-degree relatives and one-fifth of extended relatives of major depressive disorder (MDD) patients had a history of depression (Zubenko et al. 2001). Also, a Meta analysis of previous twin studies proved that major depressive disorder has a heritability of 0.33 (Wurtman 2005). Furthermore, various studies about serotonin, a neurotransmitter that is involved in emotion regulation, have been done; these studies showed that some alterations to the genes that encodes for serotonin transporters are associated with the onset of depression (Wurtman 2005). In addition, major antidepressant drugs target these "serotonin transporters" to control the onset of depression symptoms (Ho 1999).
Figure 4. The graph shows that various behavioural disorders, such as depression, are heritable.

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