A massive study of twins in Virginia suggests that three genetic factors likely underlie major depression, researchers reported Wednesday in the JAMA Psychology.

In reviewing data from 7,500 twins in the Virginia Adult Twin Study of Psychiatric and Substance Abuse Disorders, researchers found three genetic factors underlying symptoms of cognitive and psychomotor depression, as well as depression symptoms related to mood and characterized by a neurovegetative state — the latter of which involves impaired sleep, appetite and concentration. These three groups consolidate a total of nine diagnostic symptoms employed by psychologists to identify the condition in patients.

In reporting their results, the researchers sought to disabuse the medical community of the assumption that just one simple genetic cause underlies major depression. Whether focusing on specific genes or the entire genome, molecular genetic studies have been designed based on that assumption, the researchers wrote.

"Our results suggest that this approach would be inaccurate," Dr. Kenneth S. Kendler, co-author of the paper, wrote. The results "raise our awareness of the widely accepted but rarely tested assumption that the criteria for DSM-IV psychiatric and substance use disorders reflect single dimensions of genetic risk."

According to the DSM-IV, major depression diagnostic symptoms include: low mood for the better part of every day; loss of interest or pleasure in daily life; change in eating, appetite or weight; sleep disruption; impaired motor activity; fatigue; feelings of worthlessness and low self-esteem; trouble concentrating; and thoughts of death or suicide.

All nine of these symptoms indicate major depression, under which lie the three suspected genetic factors, the researchers said.

"[The] results from the current analyses should be viewed in light of findings that DSM criteria for alcohol dependence, antisocial personality disorder, and conduct disorder reflect multiple diverse genetic factors," Kendler wrote.

According to the National Institutes of Health, NIH, 6.7 percent of American adults would at some point in their life experience major depression persisting for at least a year, with 30.4 percent of those cases classified as "severe" major depression.

The researchers suggested future studies be designed to assess and analyze the three genetic factors individually rather as a whole. The researchers were affiliated with the Virginia Institute for Psychiatric and Behavioral Genetics and Virginia Commonwealth University in Richmond, with partial funding from the National Institutes of Health.