Bullied children may end up with scars that last long after their school days, according to a new twin study that found that kids who are victimized by their peers are more likely to have changes in the expression of a gene involved in mood regulation compared to their identical twin siblings who weren't.

"Since they were identical twins living in the same conditions, changes in the chemical structure surrounding the gene cannot be explained by genetics or family environment," researcher Isabelle Ouellet-Morin said in a news release. "Our results suggest that victimization experiences are the source of these changes."

Researchers at the Université de Montréal say that the change in gene expression can make bullied children more vulnerable to mental health problems as they grow older.

Ouellet-Morin and her team studied 28 pairs of identical twins born between 1994 and 1995 in the British Environmental Risk (E-Risk) Longitudinal Twin Study. One twin in each of the 28 pairs had been a victim of bullying by peers while the other had not.

Researchers had analyzed the children's DNA methylation of SERT, a gene that's responsible for regulating serotonin. Serotonin is a neurotransmitter involved in mood regulation and depression.

The findings published in the journal Psychological Medicine, reveal that twins who were victimized by peers had higher SERT DNA methylation at age 10 compared with their non-victimized twins. Furthermore, while children with higher SERT methylation levels secrete less of the stress hormone cortisol, they had more problems with social interaction and aggressive behavior.

Ouellet-Morin and her colleagues said that these changes could make bullied kids more vulnerable to mental health problems in the future.

"Many people think that our genes are immutable; however this study suggests that environment, even the social environment, can affect their functioning," Ouellet-Morin said.

"This is particularly the case for victimization experiences in childhood, which change not only our stress response but also the functioning of genes involved in mood regulation," she concluded.