July 27, 1999 by Linda Sage
It's like sitting under a cloud when everyone else is in the sun. Or watching a movie in black and white when all your friends are seeing color. Depression takes the pleasure out of life and interferes with living. It jeopardizes jobs, marriages and parenting and can lead to suicide or illness.
Because about twice as many women as men suffer from depression, most studies of the disorder's inheritance have focused on women. Researchers in St. Louis, Chicago and Australia now have studied depression in a community sample of 2,662 pairs of same-gender and mixed-gender twins. Stressful events such as the death of a spouse or loss of a job were the major causes of depression in both the men and the women, they found. However, genetic factors were more likely to have contributed to depression in the women than in the men. Surprisingly, this appeared to be true for mild as well as severe depression.
"So women who are related to a person with major depression seem to be more vulnerable to developing depression themselves than men who come from a family with depression," says Laura J. Bierut, M.D., assistant professor of psychiatry at Washington University School of Medicine in St. Louis. "And the transmission of depression appears to be due to the genes a family shares rather than to the family environment. It's not that a mother who is depressed teaches a daughter how to be depressed."
Bierut was lead author of the report, which appeared in the June 1999 issue of Archives of General Psychiatry. Grants from the National Institutes of Health, the U.S. Public Health Service and the Australian National Health and Medical Research Council funded the research.
Studying the incidence of a disorder in twins is one way to tease out genetic factors from environmental influences. Genes are likely to play a role if a disorder occurs more often in both members of identical twins, who have the same genes, than in both members of fraternal twins, who have only half of their genes in common.
In the early 1980s, a large number of Australian twins were listed in the Australian National Health and Medical Research Council Twin Registry. They were at least 18 years old at that time. Between 1992 and 1993, each member of 2,685 twin pairs was interviewed over the telephone. The highly trained lay interviewers used a list of questions designed to assess whether the subjects had ever experienced symptoms of various psychiatric disorders, including depression. Each twin was unaware of how his or her twin responded.
Bierut and colleagues used the data from these interviews for their analyses. Because they excluded people who suffered from bipolar (manic) depression rather than the more common unipolar depression, they ended up with data from 2,662 pairs of twins.
Genes vs. environment
The researchers analyzed the data using several definitions of depression. The broadest included people who had been depressed for at least two weeks at least once during their life. This applied to 24 percent of the men in the sample and 31 percent of the women. The second included only people whose depressive episodes had hampered their ability to perform daily tasks or who had sought treatment for depression. This applied to 16 percent of the men in the sample and 22 percent of the women. The strictest definition, which required at least six symptoms of depression in an episode that persisted for at least four weeks, applied to 3 percent of the men in the sample and 9 percent of the women.
Looking at whether depression had occurred in both or only one twin of a pair enabled the researchers to estimate the heritability of the disorder -- the degree to which genes contribute. For the women in the sample, heritability ranged from 36 percent to 44 percent. For the men, the range was 1 percent to 24 percent. "So in men, depression was only modestly familial," Bierut says, "which means that individual environmental factors play a larger role in the development of male depression."
This result doesn't necessarily mean that men are more likely to become depressed after a stressful event than are women, Bierut says. "But when they do develop depression, it is more likely to be due to individual stressors than to genetic factors," she explains.
Surprisingly, the estimates of heritability were similar no matter which of the three definitions of depression was used. "People have thought that severe depression is genetic depression whereas mild depression is environmental," Bierut says. "We didn't find any evidence for that."
The researchers now are investigating the inheritability of alcoholism, which occurs in men more often than in women. Their long-term goal is to understand the relationship between alcoholism and depression.
Note: Fore more information, refer to Bierut LJ, Heath AC, Bucholz KK, Dinwiddie SH, Madden PAF, Statham DJ, Dunne MP, Martin NG, "Major Depressive Disorder in a Community-based Twin Sample," Archives of General Psychiatry 56, 557-563, June 1999.
The Washington University researchers (Bierut, Heath, Bucholz and Madden) collaborated with researchers at Finch University of Health Sciences, The Chicago Medical School (Dinwiddie), Queensland Institute of Medical Research (Statham and Martin) and Queensland University of Technology (Dunne).
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