Researchers have been unable to find a single cause for depression, as there seem to be many possible factors.
Most likely, depression is caused by a combination of genetic, biological, environmental, and psychological factors.
Causes of depression
Genetics (family history): If a person has a family history of depression, he or she may be more at risk of developing it. Chemical abnormalities of the brain that cause depression may be inherited. However, depression may also occur in people who don’t have a family history of depression.
Chemical imbalance: The brains of people with depression look different than those who don’t have depression. Also, the parts of the brain that manage your mood, thoughts, sleep, appetite, and behavior don’t have the right balance of chemicals.
Hormonal factors: For women, menstrual cycle changes, pregnancy, miscarriage, postpartum period, perimenopause, and menopause may all cause a woman to develop depression.
Psychological factors: There are many theories about what kinds of life events and personality traits can cause a person to suffer from depression; for example, loss of a loved one, poor early mother-child interactions, low self-esteem, and anger turned inward.
Stress: Stressful life events such as trauma, a bad relationship, work responsibilities, caring for children and aging parents, abuse, and poverty may trigger depression in some people.
Medical illness: Dealing with serious medical illnesses like stroke, heart attack, or cancer can lead to depression.
A personal or family history of mental problems, depression, suicide, alcoholism or drug abuse.
A weak social support system: not being married/having a partner, having few family members or friends, having a job where you work alone.
There are many medical conditions, too, that can cause depression, including the following:
Pain that doesn’t go away with treatment
Abnormal levels of calcium, sodium, or potassium in the blood
Not enough vitamin B12 or folate in your diet
Too much or too little thyroid hormone
Too little adrenal hormone
Side effects of certain medicines
Depressive illnesses are disorders of the brain. Brain-imaging technologies, such as magnetic resonance imaging (MRI), have shown that the brains of people who have depression look different than those of people without depression. The parts of the brain involved in mood, thinking, sleep, appetite, and behavior appear different. But these images do not reveal why the depression has occurred — and they also cannot be used to diagnose depression.
Some types of depression tend to run in families. However, depression can occur in people without family histories of depression too. Scientists are studying certain genes that may make some people more prone to depression. Some genetics research indicates that risk for depression results from the influence of several genes acting together with environmental or other factors.
In addition, trauma, loss of a loved one, a difficult relationship, or any stressful situation may trigger a depressive episode. Other depressive episodes may occur with or without an obvious trigger.
Other illnesses may come on before depression, cause it, or be a consequence of it. But depression and other illnesses interact differently in different people. In any case, co-occurring illnesses need to be diagnosed and treated.
Anxiety disorders, such as post-traumatic stress disorder (PTSD), obsessive-compulsive disorder, panic disorder, social phobia, and generalized anxiety disorder, often accompany depression. PTSD can occur after a person experiences a terrifying event or ordeal, such as a violent assault, a natural disaster, an accident, terrorism or military combat. People experiencing PTSD are especially prone to having co-existing depression.
Alcohol and other substance abuse or dependence may also co-exist with depression. Research shows that mood disorders and substance abuse commonly occur together.
Depression also may occur with other serious medical illnesses such as heart disease, stroke, cancer, HIV/AIDS, diabetes, and Parkinson’s disease. People who have depression along with another medical illness tend to have more severe symptoms of both depression and the medical illness, more difficulty adapting to their medical condition, and more medical costs than those who do not have co-existing depression. Treating the depression can also help improve the outcome of treating the co-occurring illness.