Genes

• Those with a family history of the condition are more likely to develop depression. Scientists have not yet found a single gene that "causes" depression; instead, many genes combine with environmental factors to cause the illness.
  
  

The Brain

• Depression clearly involves the brain. This is apparent because, compared to healthy individuals, those who have depression have certain brain structures that are smaller than average.
  
  

Chemical Imbalance

• Depression also arises due to chemical imbalances in the brain. The exact relationship between brain chemicals and major depression is unclear; however, it may be that chronic chemical dysregulation damages brain structures and thereby results in depression.
  
  

Serotonin Theory

• Serotonin is one important neurotransmitter that is often dysregulated in cases of depression. Medications such as selective serotonin re-uptake inhibitors (SSRIs) are effective treatments for the disorder.
  
  

Cortisol

• The stress hormone cortisol also influences the development of depression. Stressful events trigger its release; chronic exposure damages the brain and dysregulates the balance of chemicals in the brain.