In addition to the chemical connection, there are several other theories about what causes depression. They include heredity, thinking patterns, life events, other health issues, medications or drugs, and lack of sunlight.

The Heredity Connection

Research has demonstrated that depression, like diabetes or high blood pressure, tends to run in families and can be passed on from one generation to another. Researchers haven't found a gene responsible for depression, and they are just beginning to understand the role that genetics plays in depression. Studies have shown that people who have relatives with depression are 2 to 3 times more likely to suffer from depression themselves. You are more prone to develop depression if your parents had major depression of if you had social anxiety disorder as a child or young adult. However, depression can also occur in people with no family history of depression.

The Thinking Pattern Connection

Many mental health experts believe negative ways of thinking can make some people more likely to suffer from depression. These include:

  • being pessimistic (having a negative and hopeless attitude)
  • having low self-esteem
  • worrying excessively
  • feeling as if you have little or no control over your life
Life Events/Stress Connections

Major stresses, such as divorce, job changes, moving to a new area, or the loss of a friend or family member, can trigger depression in some people. It's normal for such difficult life events to be overwhelming, causing sadness and grief. Although grief is a painful experience, most people recover. A grief reaction that lasts for more than 2 months or is unusually severe may progress into an episode of major depression. Experts estimate as many as 25% of people who experience a major loss may develop clinical depression.

Some experts believe that traumatic events experienced as children or teenagers, such as loss of a parent or sexual, physical, or emotional abuse, can trigger depression in young people; and, having had depression at an early age, they may be at an increased risk for depression later in life.

The Health Connection

Depression can be a reaction to a serious medical problem. Some illnesses cause pain or restrict the activities in which people can participate. Some illnesses are long-lasting and difficult to live with. Frustration and stress from medical illnesses can lead to depression. In addition, some types of health problems can cause brain chemistry changes that can lead to depression. Experts estimate that clinical depression may occur in as many as a third of those who have other medical problems. For some types of medical problems, the rates of depression are even higher.

Depression is common with neurological illnesses such as multiple sclerosis, Parkinson's disease, and stroke. Thyroid problems and Cushing's disease, an endocrine disorder, have been linked to depression. Rheumatoid arthritis and systemic lupus may also cause depression. In addition, depression is associated with chronic medical conditions such as cancer, diabetes, and heart disease. It may affect the chances of survival for those who have had a heart attack.

The Sunshine Connection

For some people, lack of sunlight, usually during late fall and winter months, can bring on depression. Many people tend to feel a little down on cloudy days. People with seasonal affective disorder (SAD) experience depression during periods of gray weather. Experts believe that sunlight causes changes in a tiny structure in the brain called the hypothalamus. For some people, these changes upset the brain chemistry and cause depression.

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