Are children today really going through puberty earlier?

In 1977, hundreds of young Italian boys and girls attending a school near Milan suddenly began growing breasts. A subsequent investigation published in the Lancet suggested that contaminated beef and poultry were the likely cause. A decade earlier, another outbreak of early puberty in seven young kids in California had been traced to a tuberculosis drug that was accidently laced with estrogen-like compounds. In a 2006 piece for the New York Times—which, incidentally, was embellished for an episode of House —I described the case of a brother and sister who began growing pubic hair before reaching kindergarten. It turned out their father was secretly applying a high-potency testosterone cream purchased from an Internet pharmacy, for supposed cosmetic and sexual performance benefits, and the cream was rubbing off onto his kids from normal daily contact.

As with infections or chemical spills, early puberty can occur in small outbreaks. But can it also happen on a larger, population-wide scale? Recently, a drumbeat of scientific publications have speculated that children today undergo puberty earlier than in decades past, spurring worry about pervasive environmental triggers like bisphenol-A (BPA), phthalates, and obesity.

But a closer look at the data suggests that fears about early puberty may be misplaced.

The concerns about widespread early puberty began in the 1990s, when a North Carolina physician assistant named Marcia Herman-Giddens wondered why many 7- and 8-year old girls appeared to be developing breasts. She organized a study in which 225 pediatricians graded the maturity of young girls' breasts and pubic areas. In a controversial 1997 Pediatrics paper, she concluded that puberty occurred earlier than in previously reported federal health studies from the 1960s.

Last year, Danish researchers compared data taken from girls from 2006 to 2008 to another cohort from 1991 to 1993 and also found the breast and pubic hair development was now occurring earlier by about one year. And in a well-publicized study released earlier this month, a team led by Frank Biro of Cincinnati Children's Hospital reported that American girls examined between 2004 and 2006 appeared to undergo puberty even earlier than reported by Herman-Giddens, prompting fears that the trend was accelerating.

Is it really possible that the process of human maturation could be changing rapidly? Identifying the start of puberty is very subjective and many studies showing earlier puberty, particularly those that focus on breast development, can be flawed and misleading. The key is to find a more reliable marker of puberty.

Thankfully, there is one.

The precise trigger for sexual maturation is unknown, but sometime during childhood, a grape-sized area of the brain called the hypothalamus decides one night that it's time to grow up. Beginning that night, the hypothalamus periodically drops a bit of a hormone called GnRH just onto the pea-sized pituitary gland, rousing it from its lifelong slumber.

The pituitary then secretes its own hormones into the circulation, ultimately activating the adrenal glands and ovaries (which make lots of estrogen) or testes (which make lots of testosterone). In girls, the first sign of puberty is typically a slight budding of the breast; in boys, it's a mild enlargement of the testicles. Over the next years, other changes arrive: pubic and underarm hair, voice deepening, the adolescent growth spurt, acne, menstruation or semen production, and so on.

With no objective blood test or scan, most experts consider breast budding and testicular growth the hallmarks of puberty's beginning. Unfortunately, those measures are very subjective—particularly for male children. Pediatricians guess the size of a boy's testicles by touch and comparison to a rosary-like string of balls called an orchidometer, which is not very accurate.