Puberty usually hits boys later than girls, at an average age of 11. But according to new research published in the journal Pediatrics, U.S. boys are now experiencing the first signs of sexual maturation — genital growth, testicular enlargement, and the appearance of pubic hair — roughly 6 months to 2 years earlier than boys at similar ages just a few decades ago.
Pediatricians now find that the earliest stage of male puberty occurs, on average, at age of 10.14 years among non-Hispanic whites, at 10.4 years among Hispanics, and at just 9.14 years among African Americans.
“All parents need to know whether their sons are maturing within the contemporary age range, but, until now, this has not been known for U.S. boys,” pediatrics researcher and study author Richard Wasserman said in a statement.
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This new research comes from the very same group that showed, almost 15 years ago, that girls in the U.S. are hitting puberty earlier than they used to as well. In a landmark paper in 1997, scholars and clinicians from the Pediatric Research in Office Settings program (PROS) showed that girls were reporting their first menstrual period and breast development 6 months to a year earlier than outdated clinical textbooks were listing as the average maturation age at the time.
That trend toward earlier puberty in girls is now widely accepted. But, until now, it hasn’t been clear whether boys were part of the earlier sexual maturity as well. That’s because boys don’t have the same kind of clear, easily measured marker of puberty onset as the first period, or menarche. For boys, the first signs of puberty appear more gradually, including those outcomes measured in this new Pediatrics study: testicle growth, penis growth, and the emergence of pubic hair. Testicular growth, for example, is much harder for the pediatrician to assess quickly by sight alone than, say, breast development is in girls. For these reasons, until now, researchers have simply had less data about puberty timing among males than among females, and researchers have been unsure whether trends for boys were similar to the now well-documented early blooming among girls.
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But in the new study, clinicians across the country collected data from 4,131 generally healthy U.S. boys, aged six to 16, who visited 144 pediatric offices in 41 states. The doctors visually examined the boys for genital development and presence of pubic hair. They also tested testicular volume using a medical device called an orchidometer: a string of beads, each one bigger than the previous one, that the doctor compares to the testicles to measure testicular size. They did not test of hormone levels.
Overall, the boys in the study appear to enter puberty 1.5 to 2 years earlier than predicted by “standard” puberty norms, which were established by a 1969 British study. Evidence from U.S. boys measured between 1948 to 1995 showed, similarly, that Americans boys then achieved first pubic-hair growth a good 6 months later than contemporary boys do.
The bigger question for researchers now is, why. No one is quite sure why boys are hitting puberty earlier than before, or, in fact, how much of a concern it might be. Among girls, early physical development has been linked to obesity and even heart disease. And researchers worry too about a possible association between exposure to certain chemicals, including some found in soy, as well as bisphenol A (found in certain plastics); both may mimic the sex hormone estrogen once in the body and potentially trigger early female puberty.
When it comes to early puberty in boys, however, those explanations aren’t as plausible. In fact, those factors for girls, “are not known to be associated with earlier development in boys and could even be theorized to have a delaying effect,” the authors write, possibly because they primarily involve the hormone estrogen, which promotes female sexual maturation but may have a dampening effect on testosterone, the driver of male sexual secondary characteristics.
There’s also no clear explanation for the stark racial and ethnic disparities in the timing of male puberty. African American boys in the study show signs of sexual maturation at younger ages, typically, than either whites or Hispanics. As early as age 6, close to 20% of black boys have signs of early-puberty genital growth — more than double the proportion among either whites or Hispanics. Overall, signs of puberty occur roughly one year earlier among blacks than among whites.
As one of the first studies to document the earlier onset of puberty among boys, the findings need to be reproduced by other groups, and investigated further to answer such questions about the role of race and the cause of the premature puberty.
What is clear, however, is that kids really are growing up “too fast,” and both parents and pediatricians need to be aware of how that sped-up development can affect everything from children’s social interactions to their health.