A new study has found that teenagers carrying common variants of three genes are more likely to show significantly different levels of antisocial behavior, depending on whether the individual grows up in an abusive or caring environment.
"We found that the three genetic variants interacted with each other and with family conflict and sexual abuse to increase the likelihood of delinquency, and with a positive parent-child relationship to decrease the risk of delinquency," explained Sheilagh Hodgins of the University of Montreal who pinoeered the study, along with Professor Kent Nilsson and his colleagues at Uppsala University in Sweden.
The study, published in the International Journal of Neuropsychopharmacology on 11th December, looked at three common gene variants simultaneously MAOA, BDNF, and 5-HTTLPR, and was the first of its kind to reveal that certain gene variants can, in positive environments, actually have a positive effect against juvenile delinquency.
"The implications of the study are incredibly important. First of all, what it shows is that it is not the gene that leads the behavior, but the gene that reacts with environment. Genes interact with each other and other environmental factors," Hodgkins said.
"The results show how fragile people are- when good things happen to the people with these gene variants, outcomes are better than average. When bad things happen to these individuals, outcomes are worse than the average.
"It's this notion that genes are altering sensitivity to the environment. Some people are more sensitive than others, and therefore the environment really matters," she continued.
The study was carried out on 1,337 high school students aged between 17 and 18 in the Swedish county of Västmanland. Each student anonymously completed a survey, which asked questions about levels of delinquency, experiences of sexual abuse, and what kind of relationship they had with their parents. In order to obtain DNA from each of the students, a saliva sample was also taken.
One of the genes implicated in the study is Monoamine oxidase A (MAOA), an enzyme that degrades neurotransmitters such as dopamine and serotonin, releasing energy in the process. Previous studies have found that some variants of the gene are more active than others, and it has been suggested that individuals found to carry the less active variant of the gene exhibit increased levels of crime and violent delinquency.
The results of this study reached a similar conclusion. "About 25% of caucasian men carry the less active variant of MAOA. Among them, those who experience physical abuse in childhood are more likely than those who are not abused to display serious antisocial behaviour from childhood through adulthood," Hodgins says. "Among females it is the high activity variant of the MAOA gene that interacts with adversity in childhood to increase the likelihood of antisocial behaviour."
Another of the genes, the brain-derived neurotrophic factor (BDNF) helps to shape neuronal plasticity. The term 'plasticity' refers to an individual's ability to adapt to its environment, with the period of maximum plasticity being during early development.
"The low expressing variants of BDNF are carried by approximately 30% of individuals and some previous studies had shown that this variant was associated with aggressive behaviour if carriers were exposed to aggressive peers," Hodgins said.
"The third gene we studied was the serotonin transporter 5-HTTLPR. The low activity variant of this gene is carried by approximately 20% of individuals. Among carriers of this low activity variant, those exposed to adversity in childhood are more likely than those who are not to display antisocial and aggressive behaviour," she continued.
The link between physical, sexual and emotional mistreatment of children and the development of psychiatric problems such as aggression, crime and borderline personality disorder has been being researched for over a decade. Findings have revealed that early exposure to stress generates molecular and neurobiological effects that alter the neural development in an adaptive way that prepares the adult brain to survive and reproduce in a dangerous world. This can result in increased violence and delinquency among young adults.
"[The report's] findings add to those from other studies to show that genes affect the brain, and thereby behaviour, by altering sensitivity to the environment," Hodgins concluded.
