A study conducted by Sylvie Mrug, Ph.D and colleagues from the University of Alabama determined that there are two risk factors associated with an increase in antisocial behavior in teenage girls: early puberty and their BFF’s bad behavior.

For the study, the researchers studied 2,607 teen girls at the ages of 11, 13,16; the girls and their parents were asked how old they were when they had their first period. Additionally, they were questioned about various problematic behaviors, such as delinquency and physical/nonphysical aggression.

16% of the girls had developed early (defined as having their first menstrual period before the age of 11 – average age of the onset of menarche is 12). The team discovered an association between early puberty and an increase in physical aggression and delinquency. Over time, the relationship between physical aggression and early puberty dissipated; however, the link with delinquency remained constant.

As for peer influence, defiant behavior in a young teen’s closest friends was linked to all common behavioral problems, the majority of which dissipated over time. While the girls did not associate with defiant friends at the age of 11, they found that early puberty made them more susceptible to the negative influence of a defiant friend.

The researchers concluded that relationally, girls who experience early puberty should be monitored by parents and other influential adults in their lives. Ideally, they suggest such monitoring should begin well before the age of 11.

The results of the study were published online by the journal Pediatrics.