Causes of Depression

Have you ever wondered what causes clinical depression? Perhaps you have been diagnosed with major depression, and that's made you question why some people get depressed while others don't.

Depression is an extremely complex disease. It occurs for a variety of reasons. Some people experience depression during a serious medical illness. Others may have depression with life changes such as a move or the death of a loved one. Still others have a family history of depression. Those who do may experience depression and feel overwhelmed with sadness and loneliness for no known reason.

What Are the Main Causes of Depression?

There are a number of factors that may increase the chance of depression, including the following:

• Abuse. Past physical, sexual, or emotional abuse can cause depression later in life.
• Certain medications. Some drugs, such as Accutane (used to treat acne), the antiviral drug interferon-alpha, and corticosteroids, can increase your risk of depression.
• Conflict. Depression in someone who has the biological vulnerability to develop depression may result from personal conflicts or disputes with family members or friends.
• Death or a loss. Sadness or grief from the death or loss of a loved one, though natural, may increase the risk of depression.
• Genetics. A family history of depression may increase the risk. It's thought that depression is a complex trait that may be inherited across generations, although the genetics of psychiatric disorders are not as simple or straightforward as in purely genetic diseases such as Huntington's chorea or cystic fibrosis.  
• Major events. Even good events such as starting a new job, graduating, or getting married can lead to depression. So can moving, losing a job or income, getting divorced, or retiring.
• Other personal problems. Problems such as social isolation due to other mental illnesses or being cast out of a family or social group can lead to depression.
• Serious illnesses. Sometimes depression co-exists with a major illness or is a reaction to the illness.
• Substance abuse. Nearly 30% of people with substance abuse problems also have major or clinical depression.

How Is Biology Related to Depression?

Researchers have noted differences in the brains of people who are depressed as compared to people who are not. For instance, the hippocampus, a small part of the brain that is vital to the storage of memories, appears to be smaller in some people with a history of depression than in those who've never been depressed. A smaller hippocampus has fewer serotonin receptors. Serotonin is one of many brain chemicals known as neurotransmitters that allow communication across circuits that connect different brain regions.

Scientists do not know why the hippocampus may be smaller in some people with depression. Some researchers have found that the stress hormone cortisol is produced in excess in depressed people. These investigators believe that cortisol has a toxic or "shrinking" effect on the development of hippocampus. Some experts theorize that depressed people are simply born with a smaller hippocampus and are therefore inclined to suffer from depression.  There are many other brain regions, and pathways between specific regions, thought to be involved with depression, and likely, no single brain structure or pathway fully accounts for clinical depression.

One thing is certain -- depression is a complex illness with many contributing factors. The latest scans and studies of brain chemistry suggest that antidepressants can help sustain nerve cells and allow them to form stronger connections that withstand biological stresses (called "neurotrophic effects").  As scientists gain a better understanding of the causes of depression, health professionals will be able to make better "tailored" diagnoses and, in turn, prescribe more effective treatment plans.