Girls are hitting puberty earlier and earlier. One recent study found that more than 10 percent of American girls have some breast development by age 7. This news has upset many people, but it may make evolutionary sense in some cases for girls to develop faster, according to the authors of a new paper published in Psychological Science, a journal of the Association for Psychological Science.
Girls who physically mature earlier tend to start dating, have sexual intercourse at a younger age, and have more sexual partners than girls who develop later. That puts them at risk of sexually transmitted diseases and makes them more likely to have a child while they’re still teenagers. These are generally seen as bad things, says Jay Belsky, of Birkbeck University in London, given that many psychologists and doctors think there are right and wrong ways to develop. But he says it makes more sense to look at development the way nature does—from an evolutionary perspective. This leads to the expectation that growing up in a risky, unstable environment—the kind that fosters an insecure rather than secure attachment of infant to mother—should accelerate pubertal maturation thus increasing the chances that one could reproduce before they die.
To test the relationship between a risky, unstable environment, as reflected in an insecure infant-mother attachment bond, and early puberty, Belsky and his colleagues used data on 373 white females from a large study of early child development sponsored by the National Institute of Child Health and Human Development. Girls in the study were followed from birth until the age of 15. At 15 months, security of attachment to mother was evaluated using a standard procedure involving separating and reuniting the baby with her mother in a university laboratory. Babies who smiled, vocalized, reached, or otherwise demonstrated appreciation that their mother was back were considered to be secure; those who avoided their mother following the separation or could not be comforted by her return were considered insecure. Pubertal development was evaluated by means of annual physical exams administered by nurses or physicians starting when girls were 9.5 years of age. Results revealed, as predicted, that girls who were insecure as babies started their pubertal development sooner—by about two to four months—than girls who were secure as babies. They also completed pubertal development sooner and had their first period earlier than girls who were secure as infants.
A risky, unstable early environment, as reflected in an insecure attachment, is not the only reason girls mature early; it’s also partly due to genetics. Environmental chemicals may also have some effect. Also, there’s been a trend over the last 150 years of girls maturing earlier, possibly because of improved nutrition. But the early rearing environment and the infant-mother attachment relationship are important influences and should not be ignored, says Belsky. He cowrote the study with Renate Houts of Duke University and Pasco Fearon of the University of Reading.
“An evolutionary biology perspective says, ‘look, the thing that nature most cares about—with respect to all living things, humans included—is dispersing genes in future generations,’” says Belsky. “Thus, under those conditions in which the future appears precarious, where I might not even survive long enough to breed tomorrow, then I should mature earlier so I can mate earlier before that precarious future might get me.” This is the evolutionary logic, according to Belsky, which led to the prediction—and now evidence—that early insecurity should be related to earlier pubertal development.