There is no single main cause of depression. The condition is triggered for different reasons in different individuals. However, some of the known risk factors and triggers for depression include:

• Familial predisposition: It has been shown that depression can run in families. Studies of patients with depression who have a twin, show that when the twin of a patient is an identical twin and therefore shares 100% of the patient's genes, that twin has a significantly greater chance of also having depression than when the twin of the patient is unidentical6.
• Life changing or upsetting events such as bereavement, bankruptcy, divorce, prolonged illness, moving house, job change, retirement, redundancy and financial worries are all potential triggers for depression.

These physiological factors may combine with genetic predisposition to increase the risk for depression further still, but the research is not conclusive. Adverse life events may also trigger addictive behaviours such as alcoholism, which can cause or aggravate depression.

• Long-term illness or painful conditions such as arthritis, cancer, heart disease, diabetes and chronic kidney disease can all lead to depression.
• Depressive personality disorders: Certain personality disorders or mood disorders that may cause low self-esteem can increase an individual's likelihood of developing depression.
• Depression after childbirth: Changes in hormonal levels after childbirth can bring on a condition called postnatal depression.
• Drug addiction is associated with increased levels of depression.
• Living alone, especially in later life may trigger depression, particulalry among those who already have a genetic predisposition for the condition.
• In many individuals, no specific cause or trigger for depression may be found.