Depression has numerous causes. Any of these things alone may not be enough to cause depression, but several of them acting together can trigger depression in some people.

There is no single gene that controls mood or causes depression. However, your genetic makeup is important, because genes influence the way your nerve pathways function. Experts believe several genes work together to create a vulnerability to developing depression.

Neurotransmitters and hormones are chemical messengers that enable nerve cells to communicate with one another. They play a role in everything your brain does, including controlling your mood and emotions. The way your body produces and breaks down these chemicals inside and outside the nerve cells may contribute to a person developing depression.

Biological processes are not the only explanation for depression. The circumstances of your life also have an important effect on the development of this disorder. For example, what was it like growing up? Were your parents generally supportive or critical? Were your relationships with other key people troubled? Factors in your development tend to influence your mood.

Among the life events that may contribute to your risk of depression include early loss and stress. For example, the death of a parent during childhood or a serious childhood illness may make it harder for you to grow and develop emotionally. Physical or sexual abuse may leave permanent emotional scars. Even stressful events that occur later in life can have a profound effect. For example, the loss of a job or marital discord can bring on depression in a vulnerable person. Researchers also believe that such experiences alter the biological response to stress in ways that may promote a depressed mood.

Many illnesses cause stress and make daily life difficult. This, in turn, can lead to depression in those trying to cope. In some cases, a medical illness or drug treatment affects the brain directly, and depression may be an accompanying feature.