The Stone Age Mind

An evolutionary explanation for humans' most puzzling behaviors

Why Are Girls Getting Their Periods So Young?

Research suggests certain parenting styles may trigger early menstruation.

The following is a guest post by my good friend, fellow Tar Heel, and researcher, Patty Kuo. She is currently a developmental psychology Ph.D. student at the University of Michigan, where her research interests include hormonal and neural correlates of parental care, and how the interrelationships in families affect parent-child relationships.

I got my period when I was 11. Though the thought of a 6th grader becoming physically able to get pregnant is the stuff of most parents' nightmares, it's not as uncommon as you might think. The national average age for a girl to get her first period is 12, and girls can start as early as age 8. Why so young? No one would argue that an 11-year-old is emotionally capable of raising a child. Why would female biology be seemingly so far ahead of psychology when it comes to sexual maturity? 

Recent controversy surrounding this topic tags a fatty and preservative-laden diet as the culprit. But a growing mound of research points to some surprising social and evolutionary influences on the timing of a girl's first period, namely parental care.

One of the most popular evolutionary theories on menarche (first menstruation) explores the various emotional and developmental effects of parental care. When parental care is inconsistent, less sensitive, and more emotionally negative, children will develop insecure attachments and opportunistic mating strategies. This, researchers argue, translates to earlier onset of puberty and menarche (Belsky et al., 1991).

One aspect of the theory that has generated a lot of buzz is the implication that divorce and paternal absence may accelerate the onset of menarche. The theory suggests that when Dad's not around, menarche is brought on by way of bio-signals that tell the body, "This is an unstable environment," or "There's a shortage of males in the population." In these scenarios, the evolutionary brain sees survival as less likely, and therefore speeds up the sexual maturation process. This increases the chances of successful mating opportunities before death. So, in this sense, earlier sexual maturation provides an adaptive advantage, allowing these individuals to pass on their genes before it's too late.

But what if the male present is unrelated, like a stepfather? The unrelated male hypothesis argues that the presence of an unrelated male should signal a reproductive opportunity, and thus accelerate menarche (Barkow, 1986). And in fact, recent research has shown that the presence of a stepfather does accelerate menarche (Ellis, 2004; Ellis & Garber, 2000). But interestingly enough, the conclusion is reversed in girls who have supportive and invested stepfathers (Jaffee et al., 2003). 

However, it’s not only about paternal investment. Attachment, characterized by the relationship between an infant and its primary caregiver (usually the mother; Ainsworth, 1979) has also been examined as a predictor of menarche timing. Infant attachment is assessed as secure or insecure depending upon the infant response to the mother's departure and return in, what's known as, the Strange Situation procedure (Ainsworth, 1978). Girls who had insecure attachments to their mother achieved menarche earlier than girls who had secure attachments (Belsky, Houts, & Fearon, 2010). This may be reflective of harsher maternal responses in the early environment, which influences future reproductive strategy (Belsky, Steinberg, Houts, & Halpern-Felsher, 2010). Meaning, if Mom is harsh, then it would be in a girl’s evolutionary interest to mature faster. This way, she could find a mate that would take care of her.

Today, girls are going through puberty earlier than ever, and the research seems to align with changes in parental investment in recent generations. Could the higher incidences of advanced female sexual maturation be a reflection of our culture's recent shift towards divorce and extended step-families? Familial structures will change over time, but one thing's for certain: It’s important for parents to remain supportive and involved in their kids’ lives. 

Patty Kuo can be reached at pkuo@umich.edu.

Rebecca Searles is a science journalist and social media editor at The Huffington Post.

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