Adopted children whose biological parents were drug abusers were at more than twice the risk for the disorder, according to study results. The study also showed that environmental effects on drug abuse were more pathogenic in individuals at greater genetic risk.
“Genetic risks for drug abuse will be more potent in those who come from disrupted family environments,” study researcher Kenneth S. Kendler, MD, director of the Virginia Institute for Psychiatric and Behavioral Genetics, told Healio.com.
Kendler and colleagues analyzed data collected on 18,115 adopted children born between 1950 and 1993, 78,079 biological parents and siblings and 51,208 adoptive parents and siblings. The data was obtained from nine nationwide Swedish databases, including hospital discharge, mortality, primary health care, prescribed drug and crime registers. Individuals adopted by biological relatives or by an adoptive parent living with a biological parent were excluded from the study.
The researchers identified drug abuse through numerous registers using International Classification of Disease codes for drug dependence. They performed logistic regression analyses to determine environmental and genetic risk factors associated with risks for drug abuse in adopted children.
Risk for drug abuse was significantly greater in the adopted offspring of biological parents with drug abuse (OR=2.09; 95% CI, 1.66-2.62), in biological full and half siblings of adopted children with drug abuse (OR=1.84; 95% CI, 1.28-2.64 and OR=1.41; 95% CI, 1.19-1.67, respectively) and in adoptive siblings of adopted children with drug abuse (OR=1.95; 95% CI, 1.43-2.65). Genetic risk factors such as having biological parents or siblings with a history of drug abuse, criminal activity and psychiatric or alcohol problems, and environmental risk factors such as having adoptive parents with a history of divorce, death, criminal activity and alcohol problems, as well as adoptive siblings with a history of drug abuse and psychiatric or alcohol problems, strongly predicted the risk for drug abuse, according to the researchers.
The results were replicated from earlier twin studies performed in Australia and the United States, showing a strong connection between genetic factors and drug abuse. The results were also consistent with a smaller US adoption study that examined 197 adopted children of parents selected for alcohol abuse, drug abuse or antisocial behavior.
“Risk for drug abuse in adopted children is increased by a history in biological parents and siblings not only of [drug abuse] but also of alcoholism, major psychiatric illness, and criminal convictions,” the researchers wrote. “Risk for [drug abuse] in adopted children is increased by disruption in the adoptive parent-adopted child bond by death or divorce but also by a range of indices of a disturbed adopted home environment and deviant peer influences such as parental alcoholism and sibling drug abuse, respectively.”
The researchers added that unlike previous studies that included intact families, the causal chain between environmental risk factors and drug abuse is not confounded by genetic relationships between family members.
Disclosure: The researchers report no relevant financial disclosures.