Why Has the Age of Puberty Declined in Recent Decades? III

Girls who grow up without their fathers in their homes tend to hit puberty earlier than girls who grow up with their fathers, possibly because father absence is an indicator of the degree of polygyny in society. Because girls who experience puberty early are more likely to start having sex and get pregnant at an earlier age and to become sexually more promiscuous throughout life, the age of menarche is an important determinant of the rest of her life. What other factors can influence it?

Apart from father absence, the risk and uncertainty in a girl’s family environment are generally associated with an earlier age of puberty. Anything that might potentially lead to a girl’s premature death at a young age has been shown to accelerate her maturation. This makes perfect evolutionary sense; if one may die any time, one should start reproducing sooner in order to have some chance of leaving children behind and achieving reproductive success. Those who live in risky and uncertain environments cannot afford to take their time maturing and to postpone reproduction. This is at least part of the reason why girls from poor families mature earlier and have children at an earlier age.

In contemporary western families, the level of conflict and tension between the girl and a parent (either the mother or the father) is often used as an indicator of risk and uncertainty in the family. Girls who have warm and supportive parents reach puberty and have children later than girls whose relationships with parents are rife with conflict and tension. However, I believe that environmental risk and uncertainty as cues to a particular reproductive strategy (known as the r-selected strategy of maturing early, reproducing a large quantity of offspring, and not investing heavily in any of them) are an entirely different cause of early menarche than father absence as an indicator of polygyny; otherwise, it cannot explain why all girls in polygynous societies on average experience puberty earlier than girls in monogamous societies. (Thanks to Jay Belsky, as usual, for pointing me to the latest scientific literature on this topic.)

Finally, before I conclude this series, I’d like to respond to a comment made on an earlier post. Regular readers of this blog know that I have a policy of never responding to comments from readers, because, frankly, as regular readers also know, most comments do not deserve a response. However, there is one particularly insightful comment on the first post in this series that I cannot resist commenting on.

Michael Perreault asks if girls who watch many divorced families on TV might experience puberty earlier. Because there were no realistic images of other human beings (like TV and movies) in the evolutionary environment, the human brain has difficulty distinguishing TV characters they repeatedly encounter on TV (their “TV friends”) with the real friends, although the latest work suggests that this tendency may be largely limited to people with lower general intelligence. If people have implicit difficulty separating TV from reality, then might girls also experience early puberty (which usually happens as a result of their own parents’ divorce) by watching divorced families on TV?

I love questions like this, because they could only come from someone who truly and deeply understands evolutionary psychology in general and the Savanna Principle in particular. Someone who doesn’t completely understand evolutionary psychology would never ask questions like this. As insightful and ingenious as Mr. Perreault’s suggestion is, however, I don’t think the Savanna Principle works in this particular instance, for a couple of reasons.

First, if anything, I would think that families depicted in TV shows are less likely to be divorced and rife with tension and conflict than real American families. It is true that, compared to the days of Ozzie and Harriet and Beaver, more and more TV shows today feature broken and reconstituted families. But I would think that the proportion of divorced families is lower on TV than in real life, and TV shows definitely underrepresent families with severe tension and conflict. Such families simply do not make entertaining sitcoms.

Second, and more importantly, as I mention in my previous post, the most likely proximate (biochemical) mechanism by which father absence triggers early menarche is pheromones transmitted to the girl from the unrelated men in the family (step-father, mother’s new boyfriend, etc.). If this is the case, then simply watching divorced families with a stepfather on TV is unlikely to trigger early menarche in a girl, even if her brain implicitly thinks that she is now living with a stepfather, because TV signals do not carry pheromones.