Thanks to all those who have answered my previous email. Here are a
couple of comments:
1) When I sent my inquiry I expected to get many replies of the form
"Look at this web site (journal article ..), it will answer all your
questions". Considering that the fact that slightly more boys than girls
are born has been known for such a long time (at least 300 years) one
would think that a great deal is known and that many studies have been
done. I also did a search on the web. Unfortunately any web search
involving the keyword "sex" returns a great many quite unrelated web
sites, and none of the useful ones was all that enlightening.
2) As for the reason for the observed inequality in birthrates, there
seem to be several:
a) Environmental reasons such as pollution, climate, living
conditions ect. Considering how long this problem has been studied I
would have expected to see some study to show a correlation between any
such factor and the sex ratio. Of course if such a relationship exists
it should hold not just in one study but in any study with the relevant
data.
b) Evolution. The argument here seems to be that there is an
evolutionary advantage to having more boys born than girls, maybe
because of the higher mortality rate of men. I don't find this argument
very convincing , mostly because one man can impregnate any number of
women, and so if anything I would expect this effect to work in the
opposite direction. Also, it would be amazing to see an evolutionary
effect to have as subtle an influence as is observed here.
c) Genetic reasons involving the X and Y chromosomes. If this is
true it appears to me that with our current knowledge of Genetics
somebody should have been able to give a convincing (and testable)
explanation for this phenomena
d) hlau wrote:
> ["...When couples have sex more often,
> fertilization is more likely to occur early in the menstrual cycle,
> which apparently increases the odds of male conception.
> Some observers believe this conjecture explains why the percentage
> of baby boys has usually increased after major wars."]
Now this is my favorite. I seem to have known this almost forever,
but I have never been sure whether or not it is really true or whether
it is an urban legend. Are there any studies that show a relationship
between frequency of intercourse and the sex of the baby? Now here is a
data set that my students could get excited about! (Pun intended)
Wolfgang
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