A new study published in Pediatrics indicates that American boys are beginning puberty earlier than previously indicated, with the first signs appearing at age 9.14 for African-American boys, 10.04 for Hispanic boys, and 10.14 for non-Hispanic white boys. What do scientists measure to determine the onset of puberty in boys?
Testes size. The earliest sign of puberty in boys is the enlargement of the testicles. Tanner staging, a method of visually assessing the development of secondary sex characteristics (breast development in girls, genital development in boys, and pubic hair growth in both sexes), uses the visual appearance of the testes to determine their length. But the new study used a more precise technique known as orchidometry to determine testicular volume. An orchidometer, also known as Prader beads, is a string of ovoid plastic or wooden beads ranging from 1 milliliter to 25 or 30 milliliters in volume. Prepubertal boys typically have a testicular volume of 1 to 2 milliliters; boys in the early stage of puberty have a testicular volume of 3 to 4 milliliters. (Adult men usually have a testicular volume between 15 and 25 milliliters.) To use an orchidometer, doctors gently pull the testicle to the bottom of a boy’s scrotum and use touch and sight to find the bead that matches it in volume.
Orchidometry is a clear improvement over visual assessments of testis size, since it takes a testicle’s three-dimensional nature into account and isn’t affected by scrotal tension. However, it has the drawback of being more invasive than a visual assessment. More reliable still is an ultrasound, which (unlike orchidometry) can’t be distorted by the thickness of scrotal skin. But since ultrasounds are even more invasive than orchidometers, researchers are unlikely to do any large-scale research on testis size using this method anytime soon. (For clinical rather than research purposes, endocrinologists sometimes prescribe testicular ultrasounds when they suspect developmental abnormalities in boys.)
Only one previous study has documented American boys’ testicular volume using orchidometry , and it was on a smaller scale than the new study, so it’s unclear whether boys are really beginning puberty earlier than they used to—we simply don’t have comparable data from earlier decades.
Though testicle growth is the first sign of puberty in boys, later signs of puberty—including penis growth, thinning of the scrotal skin, changes in scrotal color, and the development of pubic hair—can and have been measured by researchers. Researchers use textbook pictures (either photographs or drawings) to determine which Tanner stage a boy is in. (The Tanner stages range from 1, prepubertal, to 5, fully developed.) Of course, there’s some subjectivity in rating phallic appearance and pubic hair thickness, for instance, which makes measuring these developments across populations difficult. Furthermore, pubic hair growth can sometimes be spurred by hormones released by the adrenal glands instead of those released by the pituitary gland in the course of puberty, which means that pubic hair appearance on its own is not a fully reliable source of information. Although puberty is associated with hormonal changes, measuring hormones in the bloodstream or urine is not considered a useful way of measuring development by most puberty researchers because the correlation between hormonal increases and physical changes is highly variable.
Ideally, puberty researchers would be able to track the physical development of a large number of boys over several years to get a better sense of the average rate of genital growth and timing of specific developments. Unfortunately, this kind of longitudinal study is highly impractical both because of its costs and because of the difficulty of finding boys whose families are willing to enroll them in a multiyear study that involves somewhat invasive measuring techniques. James Tanner did do a longitudinal study of boys to determine the contours of his famous Tanner stages—but that’s because he had a captive sample of 228 institutionalized orphan boys in the 1960s. Performing research on such populations is highly frowned upon today.
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Explainer thanks Laura Bachrach of Stanford University, Frank M. Biro of Cincinnati Children’s Hospital, and Marcia Herman-Giddens of the University of North Carolina.