G.L.Piggy [at] gmail.com
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The usual caveats apply, but there is a connection between lead exposure and delayed puberty, and it would be interesting to investigate how much of a factor the decrease in environmental lead has led to the decrease in age at puberty onset. It could be entirely possible, probable in fact, that the lead exposure vector runs in the opposite direction of the other common explanations for earlier puberty – other chemicals in the environment, absent fathers, urbanism, obesity/diet, and stress.
The National Institutes of Health conducted research on over 700 girls:
The researchers analyzed data on blood drawn from more than 700 girls ages 6 to 11. They found that girls with elevated levels of lead (at or above five micrograms of lead per deciliter of blood) were 75 percent less likely than girls with low levels of lead to have key adolescent hormones at levels that are associated with the beginning of puberty. In girls with elevated levels of both lead and cadmium, this pattern was even more pronounced.
In the current study, researchers followed more than 400 boys between the ages of 8 and 9 years over the course of three years. At the start of the study, over one quarter (28%) of the boys had levels of lead that were half of the threshold indicated by the CDC, or 5 micrograms/dL. By the time they had reached the age of 12 years, 90% of them had at least one developmental sign (i.e., changes in voice, body hair) indicating that they were entering puberty. However, boys who had elevated levels of lead were less likely to display these signs, with delays averaging 6 to 8 months. [emphasis added]
There has been an uptick in recent years of research of the link between lead and puberty: girls in South Africa, boys and girls in Egypt, white, black, and Hispanic American girls, and in Swiss female mice. As always, this is a nature/nurture issue. Puberty timing is 50-80% heritable.