Current theories regarding the risk factors and causes of clinical depression can be broadly classified into two categories, Physiological and Sociopsychological:

Physiological causes

Genetic predisposition

The tendency to develop depression may be inherited; there is some evidence that this disorder may run in families, though biological and environmental factors may also be responsible. A 2004 press release from the National Institute of Mental Health declares "major depression is thought to be 40–70 percent inheritable, but likely involves an interaction of several genes with environmental events".[17]

Neurological

Many modern antidepressant drugs change levels of certain neurotransmitters, namely serotonin and norepinephrine (noradrenaline). However, the relationship between serotonin, SSRIs, and depression usually is typically greatly oversimplified when presented to the public, though this may be due to the lack of scientific knowledge regarding the mechanisms of action.[18] Evidence has shown the involvement of neurogenesis in depression, though the role is not exactly known. Recent research has suggested that there may be a link between depression and neurogenesis of the hippocampus.[19] This horseshoe-shaped structure is a center for both mood and memory. Loss of neurons in the hippocampus is found in depression and correlates with impaired memory and dysthemic mood. That is why treatment usually results in an increase of serotonin levels in the brain which would in turn stimulate neurogenesis and therefore increase the total mass of the Hippocampus and restores mood and memory, therefore assisting in the fight against the mood disorder.[citation needed]

Medical conditions

Certain illnesses, including cardiovascular disease,[20] hepatitis, mononucleosis, hypothyroidism, and organic brain damage caused by degenerative conditions such as Parkinson disease, Multiple Sclerosis or by traumatic blunt force injury may contribute to depression, as may certain prescription drugs such as hormonal contraception methods and steroids.