by Peter Frost
Whom will she choose? (source)
There has beenmuch talk about two findings from a recent study: (a) boys reach puberty atdifferent ages in different ethnic groups and (b) boys are reaching pubertyearlier now than in the recent past.
[…] we found significant differences in the age ofonset of stage 2 genital and pubic hair growth between African American boys ascompared with white and Hispanic boys and transition to testicular volumes ≥4mL (but not 3 mL). The meaning of this finding is unclear, as no existingstudies inform differences in mean testicular size at given ages, byrace/ethnicity, and sexual maturity stage; or in racial/ethnic differences inthe rate of advancement through the Tanner stages over time. (Herman-Giddens, 2012)
The secondfinding is new: We observed that onset of secondary sexualcharacteristics in US boys as seen in office practice appears to occur earlierthan in previous US studies and the 1969 British study commonly used forpubertal norms. […] White boys in our study entered stage 2 genital growth 1.5years earlier than the British boys (10.14 vs 11.60 years of age).
[…] These data are consistent with recent trendsfrom 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 nowthan 15 years ago. (Herman-Giddens, 2012)
The authors putthe cause down to “exposure to chemicals, changes in diet, less physicalactivity, and other modern lifestyle changes and exposures.” In an article forCNN, the lead author elaborated:
“The changes are too fast,” Herman-Giddessaid. “Genetics take maybe hundreds, thousands of years. You have to lookat something in the environment. That would include everything from (a lack of)exercise to junk food to TV to chemicals.” (Wilson, 2012)
Yes, new geneticvariants take time to appear through mutation. But variants for early pubertyalready exist in the population. Natural selection has only one thing to do:increase the proportion of people with those variants. And that can happen witheach passing generation. In any givenpopulation, almost all variability in male pubertal timing is genetic. This wasthe conclusion of a Swedish twin study:
The heritability was 0.91 for age at onset of growthspurt and 0.93 for age at peak height velocity in this Swedish cohort of maletwin pairs. Of interest is that these heritability estimates are almost thesame 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, werefound in a Polish twin study.(Silventoinen et al., 2008)
There is thusplenty of genetic variation for selection to act on. No need to wait for newmutations. 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 malesto aggression, impulsivity, sexual compulsivity, and lifetime number of sexpartners and in females to paternal divorce, father absence, and early menarche(Comings et al., 2002). It is likely that these alleles also influence malepubertal timing, but research on this point is lacking—apparently because it isdifficult to find a marker for pubertal maturation among boys that isas salient as age at menarche among girls (Ge et al., 2007). Early male pubertythus 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 favorone strategy or another, depending on the current cultural environment.
Is natural selection now favoring the “cads” over the “dads”? That mightbe what’s happening. As sexual relationships become less stable and shorter-term,women will ignore men who are oriented towards stable, long-termrelationships. This was the conclusionof a study directed by Kruger et al. (2003) at theUniversity of Michigan’s Institute for Social Research:
In the study, 257 women in college were asked to read passages fromScott’s novels. Each read a paragraph describing a dark hero and one describinga proper hero. Then the women were asked which type of man they would preferfor a relationship. As predicted by the cad-dad theory of human mating strategies, the womenpreferred the proper heroes for long-term unions. When asked which characterthey would like to see their future daughters choose, they also selected properheroes. But when asked who appealed to them most for short-term affairs, thewomen turned to the dark heroes: the handsome, passionate and daring cads(Duenwald, 2003).
ReferencesComings,D.E., D. Muhleman, J.P. Johnson, & J.P. MacMurray. (2002). Parent-daughtertransmission of the androgen receptor gene as an explanation of the effect offather absence on age of menarche. ChildDevelopment, 73, 1046-1051. Duenwald,M. (2003). For a good time, well, don’t call dad, University ofNebraska-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 EnvironmentalInfluences on Pubertal Timing: Results From Two National Sibling Studies, Journal of Research onAdolescence, 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 inboys: Data from the Pediatric Research in Office Settings Network, Pediatrics, 130, e1058-e1068.
http://pediatrics.aappublications.org/content/130/5/e1058.full.pdf+htmlKruger, D.J., M.Fisher, & I. Jobling. (2003). Proper and dark heroes as DADS and CADS. Alternativemating 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 itsassociations 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