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).
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