The biological factors those effect on depression including; genes, hormones, and brain chemicals. It is suggested that there is a trait that codes for depression in humans, as depression often runs in families. However, this cannot be confirmed because the prevalence within families could be due to the environment created as results of the disorder. According to findings of Nurnberger and Gershon in 1982, the MZ twins had a concordance rate of %65 (both twins are had depression) while the DZ twins had a concordance rate of 14%. According to Sullivan (2000), the MZ twins were more than twice as likely to develop depression if their co-twin had it than DZ twins. On average, the genetic influence for developing depression was between 31% and
