A UAB study shows early maturing in adolescent girls can increase aggressive and delinquent behavior.

Findings from a University of Alabama at Birmingham study published Dec. 9 in the journal Pediatrics show that adolescent girls who experience their first menstrual cycle prior to age 11 reported more delinquent and physically aggressive behavior.

By age 16, the effect of early puberty on physical aggression disappeared, but these girls still reported more delinquent behavior than those who did not experience early puberty.

“Delinquency and aggression put adolescents at risk for many negative outcomes in the future, including lower educational achievement, substance abuse, depression and problems in relationships,” explained the study’s lead author Sylvie Mrug, Ph.D., associate professor in the UAB Department of Psychology. “Thus it is important to understand how these problem behaviors develop and how pubertal timing and friends’ behavior — among other variables — contribute to them.”

Mrug and colleagues interviewed more than 2,600 girls and their parents three times between the ages of 11-16 to examine how early puberty onset and best friends’ problem behavior, like talking back to adults, lying, cheating and not being nice, contributed to delinquency and different types of aggression over time.

Girls who had a best friend with more problem behavior reported more delinquent and aggressive behavior at age 11, but these effects mostly dissipated by age 16.

“This suggests that negative peer influences from best friends at age 11 are short-lived, perhaps because best friends change as children enter middle school,” Mrug said. “The most interesting finding was that girls who experienced early puberty reported more delinquent behavior if their best friend was more deviant.”

The results suggest that early maturing girls are more vulnerable to negative peer influences.

“It is important for parents and other adults to monitor who the friends are and what the girls do with their friends,” Mrug said. “Of course this is important for all children and adolescents, but it may be even more critical for girls who mature early, as they are more vulnerable.”

Another result is that early puberty and best friends’ behavior at age 11 do not put girls on trajectories of long-term problem behaviors, according to Mrug.

“These influences can be short-lived, and this may give hope to families dealing with such issues,” she said.

Mrug says it is important to have more studies that follow girls and boys from childhood through adolescence and into adulthood to see how much different risk factors matter in the long-term. Further study is needed to examine other relevant risk and protective factors such as social support or parenting influences.

The sample of girls came from a variety of racial and ethnic backgrounds, and results showed that most of the relationships between early puberty, friends’ behavior and aggression and delinquency are the same across race and ethnicity.