BIOLOGICAL THEORY

This theory states that depression is caused by malfunction of chemicals in our brain. This issue starts if the hippocampus, a small part of the brain that is vital to the storage of memories, appears to be smaller in people with a history of depression than in those who've never been depressed. This will cause a smaller amount of seratonin receptors. Serotonin is a calming brain chemical known as a neurotransmitter that allows communication between nerves in the brain and the body. It’s also thought that the neurotransmitter norepinephrine may be involved in depression. In general, it states that our body gets depression by an inappropiate balance of chemicals.

http://www.psychologyinfo.com/depression/index.html