For girls, the age of puberty has been falling since the 19th century. The same period has seen a similar decline for boys (source)
In the United States and other Western countries, girls have been reaching puberty at earlier and earlier ages. A recent longitudinal study has examined this trend in white Americans born between 1928 and 1992. Its conclusion? Girls are reaching puberty earlier because of an interaction between a lifestyle factor and a pre-existing genetic predisposition:
Our data also show, for the first time, that the effect of menarche SNPs on prepubertal BMI was stronger in children born more recently compared to those born earlier in the century, thereby suggesting that the developmental genetic susceptibility to elevated BMI may have only been ''uncovered'' in the more obesogenic environments of the recent past. (Johnson et al., 2013)
For the study's authors, the lifestyle factor is that girls are eating more, exercising less, and accumulating more body fat. Because fatty tissue is a significant source of estrogen, an increasing percentage of body fat tends to hasten puberty in young girls (Frisch & Revelle, 1970; Frisch & McArthur, 1974; Kaplowitz et al., 2001; Siiteri & MacDonald, 1973). This effect is stronger in girls with a certain genetic background:
It is possible that over the examined time period, individuals with higher genetic burden for accelerated sexual development are for the first time ''allowed'' by liberalization of the environment to alter dietary intake and energy expenditure to support their genetic potential for rapid weight gain and earlier sexual development. (Johnson et al., 2013)
But why are boys too maturing earlier?
In boys, body fat is not linked to early puberty. In fact, there seems to be a negative correlation, perhaps because fatty tissue is a significant source of estrogen (Wang, 2002). Overweight boys often present signs of disrupted male sexual development, e.g., breast budding, higher voice pitch, etc.
Yet boys likewise are reaching puberty at an earlier age. This is the conclusion of a recent American study:
We observed that onset of secondary sexual characteristics in US boys as seen in office practice appears to occur earlier than in previous US studies and the 1969 British study commonly used for pubertal norms. […] White boys in our study entered stage 2 genital growth 1.5 years earlier than the British boys (10.14 vs 11.60 years of age).
[…] These data are consistent with recent trends from other countries, such as Denmark, Sweden, Great Britain, Italy, and China. For example, urban Han Chinese boys achieve a testicular volume of ≥4 mL (13% by age 9) and spermarche earlier than studies conducted several decades ago; Danish boys achieve a testicular volume >3 mL more than 3 months earlier now than 15 years ago. (Herman-Giddens, 2012)
This trend has also been observed in the age when a boy's voice begins to change:
According to records kept by the Leipzig choir, the most common period of voice breaking for male singers in the mid-18th century was between 17.5 and 18.5 years of age (Daw, 1970); in contrast, children enrolled in the Copenhagen Municipal Choir School from 1994-2003 had a median age of voice breaking of 10.4 years (Juul, Magnusdottir, Scheike, Prytz, & Skakkebæk, 2007), which is consistent with the choir's subjective reports of difficulty retaining children as singers past the age of 12 or 13 years. (Mendle & Ferrero, 2012).
This is a challenge for Occam's Razor, and the task is no easier if we look at other possible causes. If the cause isn’t a higher proportion of body fat, could it be a higher level of estrogens and estrogen-like substances in the environment? (see earlier post). Yes, that might hasten puberty in girls and increase accumulation of body fat. But in boys it would delay puberty by offsetting the rising level of male hormones.
In trying to figure out the causal chain of events, we should keep in mind that the relationship between body fat and age of puberty runs in both directions. On the one hand, estrogen from body fat lowers the age of puberty in girls. On the other hand, earlier puberty increases ovarian production of estrogen, which in turn stimulates deposition of body fat, particularly on the hips, buttocks, and breasts (Van Lenthe et al., 1996). So perhaps some unknown factor is causing earlier sexual development in both sexes and thus greater deposition of body fat in girls.
A response to social cues?
This unknown factor might be something in the social environment. As Hawley (2011) argues, humans unconsciously monitor their social environment for reproductive opportunities and accordingly speed up or slow down their pace of sexual development:
[...] human children, especially girls, may be sensitive to their early socioecological conditions in ways that entrain development toward either a faster (earlier pubertal maturation, more sexual partners, less stable relationships) or slower (later pubertal maturation, fewer sexual partners, more stable relationships) life history strategy.
With the transition to post-traditional societies, there has been an increase in the erotic stimuli that preteens encounter in their surroundings:
Common in traditional societies are adult-supervised adolescent initiation ceremonies (Schlegel & Barry, 1980) that are designed to commemorate the transition from childhood to adulthood and inculcate the adolescent with adult values, duties, behaviors, and sex roles associated with the culture (Schlegel, 1973). That is, these adolescents are taught adult sex roles by adults. We now appear to have a complete turnaround. In modern, Western cultures, adolescents derive sexual relationship expectations from television, cable, music, purveyors of racy lingerie (who target teenage girls), and pornography that they can now access on the Internet and thereby carry around on their cell phones. (Hawley, 2011)
Erotic imagery in particular is today available to a degree that was impossible not so long ago. Young boys and girls have virtual access to an endless supply of picture-perfect sexual partners. Whatever the media—films, TV, magazines, the Internet—we're exposed to images that can stimulate sexual desire as efficiently as what normally exists in the real world. More so, in fact. These images are ‘supernormal’ stimuli.
To date, only one study has looked into possible relationships between erotic imagery and pubertal timing:
The aim of this study was to investigate associations between pubertal timing and boys' Internet use, particularly their viewing of pornography. We used a sample comprising of 97 boys in grade 8 (M age, 14.22 years) from two schools in a medium-sized Swedish town. This age should be optimal for differentiating early, on-time, and later-maturing boys. Boys responded to self-report questionnaires on their Internet use and pubertal timing. Early, on-time, and late-maturing boys did not differ in terms of most Internet activities. However, early maturers reported downloading and viewing pornography more often than the other boys did (p<.001). (Skoog et al.,2009)
Admittedly, the arrow of causality might point in the other direction, i.e., early maturing boys have a stronger sex drive and thus a greater interest in porn. This was, in fact, the authors' explanation. We should also remember the well established correlation between early puberty in girls and the absence of a father in the home. It was long thought that father absence triggers early puberty in girls. In fact, a twin study has shown a genetic cause: absent fathers tend to have genes that favor earlier sexual development in their progeny (Mendle et al., 2006).
One might also object that the decline in the age of puberty began long before the Internet. Before the Internet, however, there were porn magazines. And before them, there were pictures garnered from art books, fashion magazines, or the lingerie sections of mail-order catalogues. One could also bring erotic images to mind by reading certain novels. Thus, modern pornography is merely the latest stage of a lengthy co-evolution between, on the one hand, improvements in photography and other imaging technologies and, on the other hand, a weakening of taboos against masturbation. At the beginning of this co-evolution, in the 19th century, masturbation was much less developed among young boys and girls as a sexual lifestyle. Visual aids were scarce and of poor quality, religious injunctions were strong, and adult supervision inside and outside the home was omnipresent.
Conclusion
The age of puberty might be declining because boys and girls are being exposed to ever more and ever better erotic imagery, but this hypothesis needs confirmation by longitudinal studies to determine which is the cause and which is the effect. Another drawback with current research is its focus on the most extreme forms of pornography, such as child porn. Yet the usual stuff is the kind that most people consume ... and in unparalleled quantities. As the authors of a recent Dutch study remarked:
[...] we can only emphasize that Dutch youth are confronted with and expose themselves to an unprecedented amount of R-rated and Xrated material in the media. Research on its consequences for adolescents' sexual socialization is largely missing but, as this study has shown, is urgently needed. (Peter & Valkenberg, 2006)
And erotic imagery isn't confined to X-rated websites or magazines. It is in fact ubiquitous in modern social environments. Girls might accelerate their sexual development by leafing through fashion magazines just as boys might accelerate theirs by viewing porn.
The erotic imagery hypothesis will have to fit the data better than rival hypotheses. Two of these, the body fat and environmental estrogen hypotheses, can explain the decline in the age of puberty for girls but not for boys. Another possible cause is better nutrition. Yet, among white Americans at least, much of this decline has happened since the 1950s—when nutrient levels were already adequate for this population. Finally, there is the possibility that puberty is happening earlier because genes that favor that developmental trajectory are spreading within the population. Modern social environments favor a reproductive strategy of early puberty, low parental investment and, especially, low paternal investment—in short, the ‘cads’ are outbreeding the ‘dads’ (see earlier post).
References
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Hawley, P.H. (2011). The evolution of adolescence and the adolescence of evolution: The coming of age of humans and the theory about the forces that made them, Journal of Research on Adolescence, 21, 307-316.
http://www.people.ku.edu/~phawley/Publications/Hawley%202011%20JRA%20Evo%20of%20Adol.pdfHerman-Giddens, M.E., J. Steffes, D. Harris, E. Slora, M. Hussey, S.A. Dowshen, R. Wasserman, J.R. Serwint, L. Smitherman, & E.O. Reiter. (2012). Secondary sexual characteristics in boys: Data from the Pediatric Research in Office Settings Network, Pediatrics, 130, e1058-e1068.
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