Puberty Starting Sooner in U.S. Boys

American boys appear to be maturing sexually at a younger age, a new study found.

Compared with studies going as far back as the 1930s, boys evaluated at well-child visits across the U.S. from 2005 to 2010 were growing pubic hair and achieving testicular enlargement 6 months to 2 years earlier, according to study author Marcia Herman-Giddens of the University of North Carolina at Chapel Hill, and colleagues.

There were some differences depending on race/ethnicity, with blacks entering puberty earlier than whites or Hispanics, the researchers reported online in Pediatrics. The study was published to coincide with the American Academy of Pediatrics' annual meeting in New Orleans.

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The observational design did not allow for an assessment of possible reasons for the results, although the authors pointed to chemical exposures and changes in diet, physical activity levels, and other lifestyle factors as potential explanations.

Further research is needed to elucidate the causes and public health implications of the trend, they said.

Recent studies have suggested that U.S. girls are entering puberty about a year earlier than they were 40 years ago, but less is known about the maturation of boys.

Herman-Giddens and colleagues analyzed data collected from the American Academy of Pediatrics' Pediatric Research in Office Settings network, the National Medical Association's Pediatric Research Network, and the Academic Pediatric Association's Continuity Research Network.

The current analysis included 4,131 boys ages 6 to 16 who underwent Tanner staging and measurement of testicular volume conducted by 212 practitioners. Boys who had chronic medical conditions or were taking medications that could affect puberty were excluded.

Half of the boys were white, 26 percent were black, and 24 percent were Hispanic.

Tanner staging is 5-stage visual method for assessing sexual maturity through the development of secondary sexual characteristics, including genital and pubic hair growth for males. Stage 1 is prepubertal and stage 5 is fully mature.

In the current study, blacks tended to mature earlier than whites or Hispanics, with no statistical differences between the latter two groups.

The average age for the onset of Tanner stage 2 genital development was 10.14 for whites, 9.14 for blacks, and 10.04 for Hispanics. The corresponding average ages for stage 2 pubic hair growth were 11.47, 10.25, and 11.43, respectively.

Whites and Hispanics entered stage 2 genital development 1.5 years earlier – and blacks 2 years earlier -- than boys in a previous study conducted in England from the 1950s to the 1970s that is commonly used for pubertal norms, according to the researchers.

And compared with previous U.S. studies, the mean age of onset for stage 2 pubic hair growth was 6 months earlier for white boys in the current study. There are limited historical data for black and Hispanic boys.

In the current study, the average age for achieving testicular volumes of at least 3 mL, which is indicative of central pubertal take-off, was 9.95 for whites, 9.71 for blacks, and 9.63 for Hispanics, with no statistical difference between the groups.

When the threshold of at least 4 mL was used, the average ages were 11.46, 11.75, and 11.29, respectively. Only the difference between blacks and Hispanics was statistically significant.

The average ages for reaching sexual maturity (Tanner stage 5) according to genital development and pubic hair growth ranged from 15.51 to 15.89, with no differences between the racial/ethnic groups.

The meaning of the differences by race/ethnicity in the onset of puberty and in testicular growth "is unclear," according to the authors, "as no existing studies inform differences in mean testicular size at given ages, by race/ethnicity, and sexual maturity stage; or in racial/ethnic differences in the rate of advancement through the Tanner stages over time."

They acknowledged that the study sample may not be representative of boys from the U.S. population and that the age calculations may lack precision because of the inability to collect the day of birth due to privacy considerations.