Wolfgang:
As I remember my course in demography many years ago, there were indications
that the prenatal mortality was excessively higher in males than in females.
An evolutionary argument would claim that whatever behavior equalizes
(approximately) the birth ratio would be advantageous. The other scrap of
data I have has to do with the pH of the vaginal tract during the normal
female cycle. As the pH varies, there is a variable survival rate of
spermatazoa with the different sex chromosomes. That goes along with the
higher frequency of intercourse theory.
Wolfgang Rolke wrote:
> 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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