cogtest

I don't know if all of these statistics are related, but I was looking at articles online and I found two that seemed like a possible explanation for earlier menarche in lower income areas. The first article said that there was a high correlation between lower income families and obesity. The other article talked about a critical weight that must be achieved before girls reaach menarche because it is triggered when the body is developed enough to carry a child. These two artcles together suggest that possibly menarche starts at a younger age in lower income areas because girls achieve the critical weight faster. Maybe this is just one factor out of many that affects this statistic.

I think the correlation between lower income families and obesity is definitely a factor that affects early menarche. However, while I think early menarche may be borne out of an evolutionary pressure, I think that the lower socio-economic classes and obesity relationship works against traditional evolutionary pressure. In evolutionary history, those who were of lower classes had access to less resources and may not have made it to reproductive age. However, with fast-food and poor eating habits, people of lower SES classes tend to eat less nutritious foods and more fast-foods. This results in more obesity and earlier menarch and, due to many factors, including the income disparity, produce more teenage pregnancies. Evolution, as it always has, is still selecting for those who eat enough to reach menarche, however, it is not selecting for those with the highest incomes, best resources, and such.
Evolutionary pressure has gotten turned around a bit in modern times, with humans outsmarting nature and creating foods high in refined sugar and usaturated oils, foods our ancestors did not eat--we are changing the way evolutionary pressures select. Early menarche is most likely just one example.

I personally believe that the link between obesity and menarche should be researched further and that may play a very large role for the discrepancy. I also think though that a major reason for this early menarche is stress, as a function of lower socio-economic status or prospects. As far as personal experience goes, most if not all, of the girls who experienced earlier menarche/pregnacy came from problem filled households or areas. To explain my confidence in this assertion, I went to an excellent public high school which was highly segregated by income; moreover, people tended to associate by neighberhoods which were of course representative of certain socio-economic scales that we were aware of. Basically, what middle school you went to indicated your financial status.

The idea that females reach menarche earlier in stressful situations actually doesn't make complete sense to me. I do think that culture plays a large part in the fact that females have children at a younger age in poverty. If a teenager has a child, that child most likely will not have the resources and educational opportunities that other children who are born to older educated parents have. As a result, this child will grow up in a stressful environment and the idea of an uncertain future causes this child to have children very early in life - it becomes a vicious cycle. However, if one of these children were to do exceptionally well in school and go on to get a good job and wait to have children until he or she was older, their child would be more "fit" and successful. So it seems that evolution should almost select for those in stressful situations to have children later in life (and as a result reach menarche later). On the other hand, the number of people who actually become successful enough to get out of poverty may be so few that this evolutionary method wouldn't be successful. With such an uncertain future it may be best to just spread genes as soon as possible...

One interesting finding is that stress will reliably increase cortisol (a steriod) levels in humans. One effect of high cortisol levels is increased weight gain -- specifically body fat. High cortisol levels cause people to both eat more AND store that energy as fat.

With Professor Whiting's latest information, it seems that early menarche is more of a bi-product of cultural differences between SES classes. A sort of chain reaction is created which links lower income families to early menarche. Lower economic status brings an increased level of stress, increasing cortisol levels and creating weight gain. This weight gain triggers menarche at an earlier age. A more in-depth study would need to be done comparing early menarche to obesity to confirm my assertions.