Depression is caused by a combination of genetic and environmental factors. Scientists are beginning to discover some of the many genes that contribute to depression. By examining family histories of mental illness and comparing the rates of depression in identical and fraternal twins, it's clear that depression and other mental illnesses have a strong genetic component.
Depression and bipolar disorder (alternating bouts of depression and mania, also called manic depression) are often considered together because they often both occur in the same family and appear to have some common genetic causes.
Heritability
Heritability refers to the extent that a trait is determined by genetic factors. According to Sullivan et al (American Journal of Psychiatry, 2000), the heritability of depression is approximately 40 percent. Therefore, almost half of the cause of depression can be attributed to genes and genetic variations in the population. "Environmental factors" account for the other approximately 60 percent. These factors may include diet, basic disposition (whether someone usually looks on the bright side or not), medication use and other factors.
The heritability of bipolar disorder is 60 percent to 90 percent, suggesting a stronger influence of genes in its cause.
Background Rsks
Approximately 10 percent to 25 percent of women and 5 percent to 13 percent of men develop depression during their lifetime. Women are two to three times more likely to develop depression than men. The mean age of onset of depression is 27 to 30 years.
The prevalence of bipolar disorder is lower; approximately 1 percent of people develop bipolar disorder during their lifetime, and the rate is similar in men and women.
Family Risks
The chance for a person to develop depression or bipolar disorder can be estimated based on who else in the family has these conditions. The stronger the family history of depression or bipolar disorder, the higher the chance for another family member to develop one of these conditions.
When a person has depression, the chance for a first-degree relative (such as a child or sibling) to develop depression is about 5 percent to 30 percent, or about two to four times the background risk. The chance is even higher when two or more first-degree relatives have depression.
When a person has bipolar disorder, the chance for a first-degree relative to develop bipolar disorder is 5 percent to 30 percent and the chance to develop depression is 10 percent to 50 percent. Offspring of two parents with bipolar have a 50 percent to 60 percent chance to develop bipolar disorder.
The chance for second-degree relatives (such as a niece or grandchild) of a person with either depression or bipolar disorder to develop one of these conditions is only slightly increased (about one or two times the background risk).
Whatever the genetic and environmental factors that contribute to depression and bipolar disorder, they seem to also relate to other psychiatric conditions and mood disorders. Relatives of people with depression or bipolar disorder also have increased risks for other conditions such as anxiety disorders, substance abuse and ADHD.
References
- SmollerJW, Finn CT. Family, twin, and adoption studies of bipolar
- Sullivan PF, Neale MC, Kendler KS. Genetic epidemiology of major
