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 indiv
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Contact: Linda Sage
314-286-0119
Washington University in St. Louis
26-Jul-1999