I'm going to jump in here. Meta-analysis has shown that approximately 80% of all cases of depression are initially caused by a major life stress (1). So "exoge

I'm going to jump in here. Meta-analysis has shown that approximately 80% of all cases of depression are initially caused by a major life stress (1). So "exogenous" factors seem to play a large role in triggering most depression. That said, not everyone develops depression following a major life stress - people have different levels of "endogenous" stress resilience/susceptibility. "Endogenous" stress resilience can be modulated, however, by "exogenous" interventions. For example, resilience can be increased by behavioral stress inoculation or even exercise, and it can be decreased by things like sleep deprivation or illness. It can also be partially inherited (i.e., if someone's parents were "exogenously" stressed, the offspring can inherit lower levels of stress resilience through "endogenous" epigenetic changes) (2). And though 80% of initial depressive episodes may be triggered by stress, this tight correlation between stress and depression does not persist as strongly after the first depressive episode. Essentially, once someone has been depressed a first time, they are then "endogenously" more susceptible to subsequent episodes of depression. So, is it exogenous or endogenous? Long story short, in most cases, it's both. Even if it doesn't start as both, it becomes both. And both "exogenous" and "endogenous" contributing factors interact in an on-going dynamic process. 1. http://hammenlab.psych.ucla.edu/pubs/05stressand.pdf http://hammenlab.psych.ucla.edu/pubs/05stressand.pdf 2. http://www.ncbi.nlm.nih.gov/pubmed/26410355 http://www.ncbi.nlm.nih.gov/pubmed/26410355