Experts assume that the causes of depression depend on various factors. It is believed that both genetic factors and biological processes, as well as psychosocial factors can all have an influence on the development of depression .
Biological processes refer to changes in the brain and the central nervous system. Studies have shown that stimuli are transmitted more slowly in people suffering from depression, and that the metabolism in the brain undergoes changes as well [1, 13].
Genetic factors impact the risk of developing depression. For example, there is an increased risk of experiencing a depressive episode if other family members have already suffered from depression. One study found a total of 102 variants of the genome that were associated with depression. 
Special negative events in a person’s life can also play a role; for instance, the death of an important person, a breakup or divorce. Traumatic events such as physical or psychological abuse can also have an impact in the development of depression .
We now know that burdensome life circumstances, such as permanent stress or loneliness, as well as certain personality traits (e.g. a pronounced pessimism) can additionally contribute to the manifestation of a depressive episode [1,7].
Other chronic or disabling physical illnesses (e.g., cancer or stroke) may also increase the risk of developing a depressive episode . Substance use disorder can also trigger depression . Other pre-existing mental health conditions, such as an anxiety disorder, may also play a role .
Women are 1,5 to 3 times more likely to develop depressive symptoms than men . While the initial onset of depression can occur at any age, one study found that a depressive episode typically first manifests in people between the ages of 20 and 30 .