Anxiety and depression are the most common mental health disorders. In the U.S., approximately 19 million people suffer from anxiety and 18 million from depression. Additionally, it is estimated that about 85 percent of Americans with depression also suffer from an anxiety disorder, and almost 54 percent of people with an anxiety disorder also have major depression.

Mental health researchers agree that the causes of depression are much more complex than the chemical imbalance theory suggests. A growing body of research points to other physiological factors, including inflammation, elevated stress hormones, immune system suppression, abnormal activity in certain parts of the brain, nutritional deficiencies, and shrinking brain cells. And these are just the biological causes of depression. Social and psychological factors—such as loneliness, lack of exercise, poor diet, and low self-esteem—also play an enormous role.
Bipolar disorder is different from depression, but it is included in this list is because someone with bipolar disorder experiences episodes of extremely low moods that meet the criteria for major depression (called “bipolar depression”). But a person with bipolar disorder also experiences extreme high – euphoric or irritable – moods called “mania” or a less severe form called “hypomania.”
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