Boys may be reaching sexual maturity earlier than ever, according to a new study that uses mortality data to estimate a young man's peak testosterone-driven phase of risky behavior.
According to this estimate, boys have been maturing about 2.5 months earlier per decade since at least the 1700s.
The result, study author Joshua Goldstein said in a statement, is that "being 18 today is like being 22 in 1800."
Girls have long been known to grow up faster than they once did. Multiple studies have found that girls hit puberty earlier than they used to, based on the age of first menstruation and breast development. (For mysterious reasons, having a brother seems to delay puberty in girls, however.)
Researchers suspect better nutrition and more body fat are among the factors in the trend toward earlier maturation. But unlike in girls, whose medical records note age at first menstruation, there is no easy way to assess the average onset of puberty in large populations of boys.
Growing up faster
To estimate puberty onset indirectly, Goldstein, a demographer at the Max Planck Institute in Germany, turned to mortality data. Across cultures, researchers have found a point at the end of male adolescence when mortality suddenly shoots up. This peak, called the "accident hump," correlates with the time of peak testosterone production in men. With high testosterone, young men are more likely to engage in risky shows of machismo and recklessness, increasing their death rate noticeably. [10 Most Destructive Human Behaviors]
Using detailed mortality data from Sweden, Denmark, the United Kingdom and Italy, Goldstein and his colleagues tracked the accident hump as far back as 1751. They found that this high-risk period (and, presumably, peak testosterone production) has been steadily shifting downward by about 2.5 months per decade. Other studies suggest puberty for girls has been arriving about 3.6 months earlier per decade since the 1800s.
The decline began before widespread industrialization and before the introduction of the automobile, two factors that might influence the teen mortality rate. Additionally, the researchers wrote, other anecdotes suggest boys have been getting more mature faster. For example, in the mid-1700s, boys in J.S. Bach's boys choir in Leipzig, Germany, aged out of the choir around age 18 when their voices changed. In 20th-century London, the average age of voice change was 13.
The study can't definitively point to a reason for the shifting accident hump, but a biological explanation seems likely, Goldstein wrote Aug. 17 in the open-access journal PLoS ONE. Increased nutrition and decreased disease take stress off the body, allowing more resources to be devoted to sexual maturation.
Early maturation raises many concerns about children growing up physically before their brains have time to catch up, but Goldstein said there are other implications as well. While boys and girls mature faster physically, they also get married, choose careers and have children later.
"The biological and social phases in the lives of young people are drifting apart," he said.