"Many people think that our genes are immutable; but this study suggested environment, even the social environment, can affect their functioning," lead author Isabelle Ouellet-Morin of the University of Montreal, said in a statement. "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."
Previous research by Ouellet-Morin found bullied children secreted less cortisol -- a stress hormone -- but had more problems with social interaction and aggressive behavior.
The current study indicated the reduction of cortisol, which occurs around the age of 12, was preceded two years earlier by a change in the structure surrounding the gene SERT, which regulates serotonin, a neurotransmitter involved in mood regulation and depression.
The study, published in the journal Psychological Medicine, involved 28 pairs of identical twins with a mean age of 10 who were analyzed separately according to their experiences of bullying by peers: one twin had been bullied at school while the other had not.
"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," Ouellet-Morin said.
"Our results suggest that victimization experiences are the source of these changes."