(CBS News) - Adopted children are twice as likely to use drugs if their biological parents used them, according a study of more than 18,000 adopted children in Sweden.
But don't discount a child's environment in the nature vs. nurture debate just yet: The same study showed that adopted children who lived with families with problems, such as divorce, death or criminal activity, also had a high risk of drug abuse.
"For an adoptee, having a biological parent with drug abuse who did not raise you doubles your risk for drug abuse," said first author Dr. Kenneth Kendler, director of the VCU Virginia Institute for Psychiatric and Behavioral Genetics, said in the press release. "But we also found an important role for environmental factors."
The study was conducted by researchers at Virginia Commonwealth University and Lund University in Sweden, who looked at 18,115 children born and adopted in Sweden between 1950 and 1993, as well as over 78,000 biological family members and over 51,000 adoptive family members.
The results, published in the Archives of General Psychiatry, showed that 4.5 percent of adoptees had drug-abuse problems compared 2.9 percent of the general population. Out of the adopted people, 8.6 percent had at least one biological parent with a drug problem, while only 4.2 percent did not have any family history of substance abuse. Abuse risk also doubled if the adopted individual and a biological sibling or half-sibling with substance abuse problems. The rate was also the same if they had adopted siblings with drug abuse history.
Kendler pointed out to CNN that the rates of drug abuse tend to be higher in the U.S. than in Sweden or other Scandinavian countries.
The most important factor, according to Kendler, was whether or not the adopted child was raised in a high-risk rather than a low-risk environment. "A bad environment can augment the effect of genetic risk on drug abuse," he said.
According to a 2008 study from the Archives of Pediatrics and Adolescent Medicine, about 120,000 children are adopted annually in the U.S. Forty thousand of those adoptions are international adoptions.