Two questions about sex:

John Leonard said:

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

Quoted message said:

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

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

essence through the mechanisms you suggested.

Bob

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Bob O'Hara

Rolf Nevanlinna Institute
P.O. Box 4 (Yliopistonkatu 5) FIN-00014 University of Helsinki Finland Telephone: +358-9-191 23743
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John Leonard <[email hidden]> wrote or quoted:

Quoted message said:

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.

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?

The most common hypothesis seems to invokes differences in sperm motility
- largely due to the approx 3% difference in mass between X-bearing and Y-bearing gametes.
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