Depression

Everyone feels sad from time to time, but depression is characterized by prolonged feelings of sadness, dejection and hopelessness. One in four women and one in six men will suffer from depression at some point in their lives.(7) Depression is a complicated illness, which can involve a number of contributing factors, such as genes, environment, diet, lifestyle, brain chemicals, psychology and personality.

A case of depressed mood can be brought on by an episode as simple as a terrible day at work or even a friend's cold shoulder. Sometimes individuals experience low self-esteem, feelings of guilt and hostility for no reason at all.  Most of the time, these feelings simply wear off. However sometimes these feelings are not easily forgotten and may persist for days or weeks at a time, interfering with the person's capacity to function normally.  For these people, depression is no longer an occasional tendency and is instead a clinical disorder.

Believed to be caused by malfunctioning neurotransmitters in the brain, clinical depression is an illness, like diabetes, ulcers or high blood pressure. One in four women and one in four men will suffer from clinical depression at some stage in their life.  However, because depression can mask itself as a physical illness (like chronic pain or fatigue) it is extremely difficult to diagnose accurately.

The heritability of major depression is estimated to be 38%, based on information from population-based twin studies. 
Sex differences exist in genetic risk factors for depression with the heritability of major depression estimated at 42% in women and 29% in men.  (13)