John Leonard wrote:

> I have read that the number of males born annually relative to that of the
> number of females, in the human population, is approximately a constant of
> 51/49. Since males are at a greater risk of death this difference might help
> promote a more equal distribution at the time of fertility.
> 
That was first shown by Laplace in the eighteenth century. He concluded 
that he was "morally certain" that the sex ratio was less than one half.
> 
> 
> My first question is this: how is this proportion maintained? I am at a loss
> to understand how it could be maintained except through feedback. If this is
> so, by what means does this feedback assert itself? Is it, for instance,
> conceivable that we unconsciously modulate the likelihood of male versus
> female birth via observations made in our brains?
> 
I think there is some evidence that this happens, but over a longer time 
scale, you will get evolution towards an idea sex ratio, because the 
rarer sex is favoured - if there are lots of males, then if you produce 
a female, it's more likely to have a mate, and produce offspring.  This 
is a classic result, first shown by Fisher over 70 years ago.  He showed 
that (under his assumptions), you should get an even sex ratio, but it's 
not too difficult to relax these to get a slightly skewed sex ratio, in 
essence through the mechanisms you suggested.

Bob

-- 
Bob O'Hara

Rolf Nevanlinna Institute
P.O. Box 4 (Yliopistonkatu 5)
FIN-00014 University of Helsinki
Finland
Telephone: +358-9-191 23743
Mobile: +358 50 599 0540
Fax:  +358-9-191 22 779
WWW:  http://www.RNI.Helsinki.FI/~boh/
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