In a new study, researchers found evidence that certain genetic variants, along with environmental factors, affect the brain, and therefore behavior.

"Evidence is accumulating to show that the effects of variants of many genes that are common in the population depend on environmental factors. Further, these genetic variants affect each other," Sheilagh Hodgins of the University of Montreal and its affiliated Institut Universitaire en Santé Mentale de Montréal, said in a news release.

Researchers studied a group of 1,337 high school students aged 17 to 18 years old in Vastmanland, a Swedish County. Participants provided saliva samples and completed questionnaires addressing delinquency, family conflict, experiences of sexual abuse, and the quality of their relationship with their parents.

The study found that three genetic variations interact with each other and with environmental factors to increase or decrease the likelihood of delinquency.

The Monoamine oxidase A (MAOA) gene is a key enzyme in the catabolism— the breaking down of complex materials and releasing of energy within an organism— of brain neurotransmitters. About 25 percent of Caucasian men carry the less active variant of MAOA, and those who experience physical abuse in childhood are more likely to display serious antisocial behavior from childhood through adulthood, compared to those who are not abused, the study found. Among women, the high activity variant of the MAOA gene, along with adversity in childhood, results in increased antisocial behavior.

Low-expressing variants of another gene, brain-derived neurotrophic factor (BDNF) are carried by approximately 30 percent of individuals. Previous research has shown that carriers exposed to aggressive peers displayed aggressive behavior.

The third gene researchers studied was the serotonin transporter 5-HTTLPR. Approximately 20 percent of individuals carry the low-activity variant of the gene. The study found that, among those with the low-activity variant, exposure to adversity in childhood led to antisocial and aggressive behavior, compared to those who were not exposed to adversity.

"Among carriers of the low activity variants of all three genes, those exposed to family conflict or sexual abuse or both reported high levels of delinquency while those who reported a positive and warm relationship with their parents reported little or no delinquency,” Hodgins said.

"These findings add to those from other studies to show that genes affect the brain, and thereby behavior, by altering sensitivity to the environment," she said.