Obesity is a vital factor behind girls reaching puberty earlier than their peers, confirms a detailed analysis.
While some researchers have argued hitting puberty early in life makes a person prone metabolic syndrome and diabetes, Emily Walvoord has found such diseases are not linked to early puberty, but obesity.
"Early puberty is one of the many outcomes of obesity," said Walvoord of ndiana University School of Medicine.
"There are clearly other factors we don't understand that have affected the timing," she added.
While childhood obesity is a growing epidemic, she suggested other factors include a possible increase in hormone-disrupting chemicals in the environment and more chronic stress in children's homes.
"It's very hard really to scientifically scrutinize the timing of puberty just by looking at these studies," Walvoord said.
She said many unanswered questions exist, none more troubling than that of the psychological impact. The long-term effect on adolescents is still unclear.
Girls who reach puberty early are more likely to experience negative body image, depression and other mental disorders.
Jane Mendle of University of Oregon said those in her field know the psychological effects of early puberty well.
"Whether or not that knowledge has been translated in the way it could or should be to the general population, I'm not sure."
Mendle said girls who mature earlier tend to land in social situations they are not psychologically prepared to handle. Because of that, they might develop psychological issues during puberty.
The findings were published in the Journal of Adolescent Health.