Why has the age of puberty declined in recent decades? I

The average age of puberty has steadily declined in most western societies in the past century. Improved nutrition is certainly one reason for the earlier age of puberty. But there is another potential cause for it.

Developmental psychologists have known for nearly two decades now that girls whose parents divorce early in their lives, particularly before the age of five, experience puberty earlier than their counterparts whose parents stay married. My fellow PT blogger (and fellow London exile) Jay Belsky was among those who made major theoretical contributions in this area nearly 20 years ago. Girls whose parents divorce early in their lives are also more likely to start having sex at an earlier age, have a larger number of sex partners, get pregnant while still a teenager, and experience divorce in their first marriage. Since the biological purpose of puberty is to mark the onset of the reproductive career, it makes perfect evolutionary sense that girls who undergo puberty at an earlier age start having sexual intercourse, have more sex partners, and get pregnant at an earlier age. (Recall the dangers of naturalistic fallacy here. Just because something makes perfect evolutionary sense does not mean it is good or desirable in any way.) But why does the presence or absence of the father at home during early childhood affect the age of puberty and thus the onset and promiscuity of sexual activity?

There are two competing explanations. One is that girls who experience puberty early are genetically different from those who experience it late. The other explanation is that girls have similar genetic makeup but respond to the environment differently, by starting puberty early or late. So which model is correct?

In the case of pubertal timing, both models are likely to be partially correct. In support of the genetic model, there is substantial evidence that a girl’s pubertal timing is largely heritable; about half of its variance is explained by genetic differences. In this model, girls who undergo puberty early are simultaneously more likely to get a divorce because of their greater tendency toward sexual promiscuity, and to pass on their early puberty–greater promiscuity genes to their daughters. Hence, girls who grow up without a father (because their mother got a divorce or was never married) are more likely to experience puberty early and to become more sexually promiscuous because they have inherited the genes that will predispose them to do so.

While evidence supports the genetic model, environmental influences can also affect the actual timing of puberty within the window set by the genes. This phenomenon is similar to other biological traits, such as height, weight, or intelligence. Height, for example, is highly heritable, so children of tall parents on average become taller than children of shorter parents; genes set the boundaries of potential adult height. Within these boundaries, however, environmental influences, such as nutrition or childhood exposure to disease, can determine the actual adult height.

The other half of the variance in pubertal timing is unaccounted for by the genes, so environmental conditions can still influence the actual onset of puberty within the window set by the genes. One of the most important early childhood influences is the absence of the father. In this model, girls who grow up without a father learn that men do not form lasting relationships with women and invest in their offspring. These girls then adopt a more promiscuous reproductive strategy of undergoing puberty early and forming short-term relationships with a large number of sex partners because they sense they cannot rely on men to form a committed relationship with them and provide parental investment in their offspring. In contrast, girls who grow up with a father at home learn the opposite lessons – that men do form lasting relationships with women and invest in their offspring. These girls then adopt a more restrained reproductive strategy of delaying their puberty and forming a committed long-term relationship with a partner who will invest in their offspring. Hence, the presence or absence of a father in a girl’s home before the age of five can explain both her age of puberty and her reproductive strategy.

So far, so good. But if you really think about it, there is one giant gaping logical hole in this environmental theory of pubertal timing. I’ll discuss what it is in my next post.