And at six months of age, those monkeys are also more resistant to weaning. "This is the time at which most infants are becoming more independent, they're exploring their environments, they're interacting and playing with peers. And what we found was that animals that had this alternative form of the mu-opioid receptor gene were not exhibiting these behaviors."
She says it's striking how these behaviors resemble addiction. "In a sense it's very similar to effects that you would see during periods of intoxication and withdrawal."
She says the next step is to find out if this link is true in people. If so, it might help in both prevention and treatment of addictions.
Barr and senior author Markus Heilig say, "If it's the case that in humans there is a common genetic link between early attachment differences and later alcohol use, this could actually provide us with strategies for early personalized prevention of alcohol use disorders."
As for treatment, other studies have shown a drug that acts on the receptor—naltrexone—appears to be more effective in alcoholics with the more sensitive form of the gene. "If an individual has a history of having an attachment disorder, or has a current attachment disorder, you might be more likely to recommend trying this kind of drug for treatment of their alcohol problem."
But the researchers add that anyone can get addicted—if they use drugs or alcohol. This gene is just one factor that has been found to increase people's' risk.
The research is also a contribution to attachment research. Disorders such as anxious attachment, are generally attributed to abuse, neglect or otherwise poor parenting in early childhood, and not as having a genetic basis.
If some differences in attachment are due to genetic variations, it could clear up something plenty of moms like Barr herself wonder about.
"This might inform us of why we might have one child that we treat one way and it exhibits one type of behavioral response to our leaving… and then on the other hand, we might have another child who—even if we treat them in a very similar way—could respond differently," she says. "Because there are not only these environmental variables and behavior of the mother that influence the development of mother-infant attachment, but also potentially a number of different genetic variables as well."
This research was published in PNAS Early online edition the week of March 31, 2008, and funded by the intramural research programs of the National Institute on Alcoholism and Alcohol Abuse and the National Institute of Child Health and Human Development.