Environmental factors within families have “potent influences” on the risk for drug abuse, new research confirms.
In a nationwide Swedish study, researchers found that the risk for drug abuse was strongly correlated in siblings and spouses after controlling for genetic factors.
The findings support earlier research by the same researchers that showed a strong influence of familial environmental factors on the risk for drug abuse in Swedish adopted children.With lead author Kenneth S. Kendler, MD, of the Virginia Institute for Psychiatric and Behavioral Genetics in Richmond, the study was published online December 10 in Archives ofgeneral Psychiatry.
Genes vs Environment
It is well known that drug abuse runs in families, but is this solely the result of genetic factors, or does the family environment contribute? In the latest study to look at this issue, the researchers reviewed data on 137,199 sibling pairs and 7561 spousal pairs from 9 Swedish databases (1961-2009). They observed in conditional logistic regression analysis that the risk for drug abuse was substantially higher in siblings of case vs control probands (odds ratio [OR], 5.29; 95%confidence interval [CI], 5.19 – 5.40).
The association for a diagnosis of drug abuse was considerably higher in same-sex pairs (brother-brother: OR, 6.37; 95% CI, 6.19 – 6.56, and sister-sister: OR, 5.37; 95% CI, 5.08 – 5.67) than in opposite-sex pairs (brother-sister: OR, 4.27; 95% CI, 4.09 – 4.46, and sister- brother: OR, 4.33; 95% CI, 4.15 – 4.52). If the proband sibling with drug abuse was male, 9.1% (95% CI, 8.9 – 9.3) of his male siblings and 3.1% (95% CI, 3.0 – 3.2) of his female siblings abused drugs. The corresponding figures when the proband was female were 6.6% (95% CI, 6.4 – 6.8) and 4.0% (95% CI, 3.8 – 4.2), respectively.
Age differences in sibships also had an influence. In sibships containing a drug-abusing proband, the risk for drug abuse in other siblings was strongly correlated with the age differences between them. The risk for drug abuse was nearly twice as great for a sibling born in the same year as a drug-abusing proband sibling as it was for a sibling born 10 years apart.
“Critically,” note the researchers, “these results isolated environmental effects because we only examined full siblings, all of whom had the same degree of genetic relationship.”
They also note that the decline in risk as a function of age difference was considerably steeper in sister-sister vs brother-brother pairs.”This has no plausible genetic explanation. Rather, these results suggest that the environmental effects impacting resemblance for DA [drug abuse] in brothers decay more slowly as a function of sibling differences in age than the environmental effects shared by sisters,” they note. After controlling for multiple factors, including age difference between siblings, having an older sibling who abused drugs conveyed a 42% greater risk for drug abuse than having a younger drug-abusing sibling.
Spouses Also at Risk
The researchers also observed that after a spouse was registered in a database for abusing drugs, his or her spouse had a 25-fold increased risk of being registered as abusing drugs in the same year; this declined to a 6-fold increased risk after 4 to 5 years and thereafter remained relatively stable.
“To summarize, our results in siblings and spouses — where we could unconfound the impact of genetic vs environmental factors — robustly replicated our previous adoption findings and prior results from twin and adoption studies, suggesting that environmental factors contributed substantially to the familial transmission of DA,” the researchers say.
“Furthermore, they illuminate the essential research task awaiting us, which is to understand how genetic and familial environmental risk factors act and interact over development to render individuals at low vs high risk for DA,” they write.