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Girls have been known to reach sexual maturity at younger ages through the centuries, but what about their male counterparts?
Researcher Joshua Goldstein at the Max Planck Institute in Germany believes he found a similar trend among maturing boys. In fact, he puts forth that males may be influenced by the same environmental factors that shape the onset of menstruation in girls, according to an analysis published in the journal PLoS ONE.
Finding trends among girls is easy since there's a starting point to sexual maturity: menarche, or when a young woman experiences her first menstrual cycle. For teen boys, however, a similar starting point isn't as clear.
This is where death comes in — well, sort of.
Universally, male adolescents have experienced a rise of mortality toward the end of their teen years — a trend known as the "accident hump." Whether it's from disease, violence or mere accident, more male adolescents die during late adolescence than their female peers. Even more, this peak also matches the time when male teens produce the most testosterone. The idea is that more testosterone equates to more risk-taking and potentially violent behaviors, which in some instances result in accidental deaths.
But since the mid-18th century, the accident hump has curiously occurred earlier and earlier in male adolescence. As such, an earlier accident hump may reflect a change in testosterone, and thus, when males are becoming sexually mature.
Sure, the association between mortality and hormones seems indirect, and Goldstein acknowledges this in his paper. But it's the other relationship — between teens' environment and biology — that's more direct and may account for the changes. Historically, both sexes gained access to more food and grew up with fewer environmental pressures associated with illness as cities blossomed in the 18th century. It's been thought these environmental allowances freed up constraints that usually hindered reproducing at younger ages.
By looking at 10-year periods of data from the Human Mortality Database from five European countries, Goldstein found the age at which males sexually mature to decrease by 2.4 months per decade between the mid-1700s and 1900s. Though other indicators of maturity, including height, don't necessarily support maturing earlier, one study on male voice changes reveals the onset of sexual maturity to occur earlier throughout the decades.
Most recently, the drop in age for sexual maturity seems to have plateaued. Instead, Goldstein's work reinforces the idea that the decrease occurred in the first place.
So why wouldn't other evolutionary factors select against risk-taking behaviors associated with the accident hump?
Goldstein speculates that risk-taking can reflect dominance, which may bolster males' mating status in certain primate species (think baboons).
Still, there's reason to be cautious about the findings. For one, the data might be affected by how they were collected. The results also rely on a variety of associations, and other factors that don't show up in historical demographic data might be at play. The current project also focused on more developed European countries, and the same trends might not be apparent in other parts of the world.
But ultimately, the results leave a gap between sexual maturity and emotional maturity in both sexes, Goldstein says in a press release. Parents may be encouraged to be more involved in their teens' lives in order to guide them against having unprotected or risky sex.