Obesity, good nutrition and chemical exposure are among the reasons girls in the U.S. might be starting their periods earlier than ever before.
Now add another possible factor to the list: a dysfunctional father.
Young girls who were exposed to high stress because of their father, but then experienced a better home after he left, sometimes reached menarche a year earlier than older sisters who lived with their father through adolescence, says a recently published University of Arizona study.
Professor Bruce Ellis worked with a researcher from New Zealand to better understand something they already knew: Younger sisters start menstruating earlier than older sisters in divorced families but not in intact families.
What Ellis and his colleague have now discovered is that, in families in which fathers struggled with drug or alcohol addiction, were abusive or involved in criminal activity, younger sisters started their periods an average of 11 months earlier than their older sisters.
Early puberty puts girls at an increased risk of breast cancer, depression and teen pregnancy, research shows.
Ellis' study looked at 68 divorced families and 93 intact families. The girls in each were several years apart in age, with the younger an average of 5 and the older about 12 when the marriage ended in the divorced families.
What influenced the start of puberty was not that the girls lived with an abusive or drug-addicted father, but that the situation changed and improved after he left.
"They are responding to that change by developing more quickly than their older sisters," Ellis said. Their bodies, he said, seem to be responding to a "window of opportunity."
Ellis said the idea is that these children might be adjusting their development to match their environment.
"In the world in which humans evolved, dangerous or unstable home environments meant a shorter lifespan, and going into puberty earlier in this context increased chances of surviving, reproducing and passing on your genes," he explained in a UA news release about the study.
Girls adopted from impoverished countries by families in developed nations are also at high risk for early puberty, said Ellis, who works in the UA's Norton School of Family and Consumer Sciences.
"The importance of this research was to show that the link is real: Exposure to fathers can actually alter their daughters' sexual development," he wrote.
Dr. Mark Wheeler, a pediatric endocrinologist and an associate professor with the UA, said the age at which girls first menstruate has declined gradually in recent history.
"Studies seem to indicate a more pronounced effect of biological factors, such as nutrition, genetics, etc., on menarche, with differences measured in years compared to effects of social factors, which tend to result in differences of months," Wheeler wrote in response to a Star e-mail about the study.
Social factors tend to include family adversity, such as father absence, low parental involvement and childhood stress or trauma, Wheeler said.
Factors for delayed menarche include older-sister presence, and parental warmth, involvement and approval.
Only 10 percent of all U.S. girls start to menstruate before age 11, with the median age in developed countries at 12.4 years, Wheeler said.