There has been much talk about two findings from a recent study: (a) boys reach puberty at different ages in different ethnic groups and (b) boys are reaching puberty earlier now than in the recent past.
The first finding is in line with previous studies:
[…] we found significant differences in the age of onset of stage 2 genital and pubic hair growth between African American boys as compared with white and Hispanic boys and transition to testicular volumes ≥4 mL (but not 3 mL). The meaning of this finding is unclear, as no existing studies inform differences in mean testicular size at given ages, by race/ethnicity, and sexual maturity stage; or in racial/ethnic differences in the rate of advancement through the Tanner stages over time. (Herman-Giddens, 2012)
The second finding is new:
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)
The authors put the cause down to “exposure to chemicals, changes in diet, less physical activity, and other modern lifestyle changes and exposures.” In an article for CNN, the lead author elaborated:
"The changes are too fast," Herman-Giddes said. "Genetics take maybe hundreds, thousands of years. You have to look at something in the environment. That would include everything from (a lack of) exercise to junk food to TV to chemicals." (Wilson, 2012)
Yes, new genetic variants take time to appear through mutation. But variants for early puberty already exist in the population. Natural selection has only one thing to do: increase the proportion of people with those variants. And that can happen with each passing generation.
In any given population, almost all variability in male pubertal timing is genetic. This was the conclusion of a Swedish twin study:
The heritability was 0.91 for age at onset of growth spurt and 0.93 for age at peak height velocity in this Swedish cohort of male twin pairs. Of interest is that these heritability estimates are almost the same as those reported from a Belgian twin study; that is, 0.93 and 0.92, respectively. Lower heritability estimates, 0.49 and 0.74, respectively, were found in a Polish twin study.(Silventoinen et al., 2008)
There is thus plenty of genetic variation for selection to act on. No need to wait for new mutations. But why would there be natural selection for earlier male puberty?
One reason is that early puberty is genetically linked to other sexual characteristics. In particular, a class of X-linked androgen receptor alleles is linked in males to aggression, impulsivity, sexual compulsivity, and lifetime number of sex partners and in females to paternal divorce, father absence, and early menarche (Comings et al., 2002). It is likely that these alleles also influence male pubertal timing, but research on this point is lacking—apparently because it is difficult to find a marker for pubertal maturation among boys that is as salient as age at menarche among girls (Ge et al., 2007). Early male puberty thus seems to be part of a “package,” or more precisely a reproductive strategy, that affects the way men go about finding a mate. Natural selection may favor one strategy or another, depending on the current cultural environment.
Is natural selection now favoring the “cads” over the “dads”? That might be what’s happening. As sexual relationships become less stable and shorter-term, women will ignore men who are oriented towards stable, long-term relationships. This was the conclusion of a study directed by Kruger et al. (2003) at the University of Michigan's Institute for Social Research:
One reason is that early puberty is genetically linked to other sexual characteristics. In particular, a class of X-linked androgen receptor alleles is linked in males to aggression, impulsivity, sexual compulsivity, and lifetime number of sex partners and in females to paternal divorce, father absence, and early menarche (Comings et al., 2002). It is likely that these alleles also influence male pubertal timing, but research on this point is lacking—apparently because it is difficult to find a marker for pubertal maturation among boys that is as salient as age at menarche among girls (Ge et al., 2007). Early male puberty thus seems to be part of a “package,” or more precisely a reproductive strategy, that affects the way men go about finding a mate. Natural selection may favor one strategy or another, depending on the current cultural environment.
Is natural selection now favoring the “cads” over the “dads”? That might be what’s happening. As sexual relationships become less stable and shorter-term, women will ignore men who are oriented towards stable, long-term relationships. This was the conclusion of a study directed by Kruger et al. (2003) at the University of Michigan's Institute for Social Research:
In the study, 257 women in college were asked to read passages from Scott's novels. Each read a paragraph describing a dark hero and one describing a proper hero. Then the women were asked which type of man they would prefer for a relationship.
As predicted by the cad-dad theory of human mating strategies, the women preferred the proper heroes for long-term unions. When asked which character they would like to see their future daughters choose, they also selected proper heroes. But when asked who appealed to them most for short-term affairs, the women turned to the dark heroes: the handsome, passionate and daring cads(Duenwald, 2003).
References
Comings, D.E., D. Muhleman, J.P. Johnson, & J.P. MacMurray. (2002). Parent-daughter transmission of the androgen receptor gene as an explanation of the effect of father absence on age of menarche. Child Development, 73, 1046-1051.
Duenwald, M. (2003). For a good time, well, don’t call dad, University of Nebraska-Lincoln, Anthropology, Raymond Hames
http://www.unl.edu/rhames/courses/current/dad-cad.htm Ge, X., M.N. Natsuaki, J.M. Neiderhiser, & D. Reiss. (2007). Genetic and Environmental Influences on Pubertal Timing: Results From Two National Sibling Studies, Journal of Research on Adolescence, 17, 767–788.
Herman-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.
Kruger, D.J., M. Fisher, & I. Jobling. (2003). Proper and dark heroes as DADS and CADS. Alternative mating strategies in British Romantic literature, Human Nature, 14, 305-317.
Silventoinen, K., J. Haukka, L. Dunkel, P. Tynelius, & F. Rasmussen. (2008). Genetics of pubertal timing and its associations with relative weight in childhood and adult height: The Swedish Young Male Twins Study, Pediatrics, 121, e885-891
http://pediatrics.aappublications.org/content/121/4/e885.full.pdf+htmlWilson, J. (2012). Boys – like girls – hitting puberty earlier, October 23, CNN
http://edition.cnn.com/2012/10/20/health/boys-early-puberty/index.html