A study shows that being intimidated by peers can alter genes involved in regulating emotions, making children more likely to experience psychological problems as they age. While many people believe that our genes are immutable, this study shows that the social environment may also affect the function of genes. For children in particular, this can alter not only their response to stress, but also the function of genes related to emotion regulation. Previous studies have shown that children who experience intimidation secrete less cortisol (the stress hormone) but have more social impairment and aggressive behavior. The current study suggests that the decline in cortisol, which occurs at about age 12, precedes by 2 years structural changes around the SERT gene, which regulates serotonin, a neurotransmitter involved in mood regulation and the onset of depression. The subjects of the study were 28 twins, an average of 10 years old, and one of each twin had the experience of being intimidated by a peer while the other did not. The analysis found that despite the twins living in the same environment, changes in the chemical structure around their genes could not be explained by genetics or family environment. The study concluded that the experience of peer intimidation was responsible for this genetic alteration.