more on birth sex ratios

[ttweed@wildrockies.org (Tony Tweedale)]

(BCC's:  sorry if this is old news to you)
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Biggar et al. 'Sex Ratios, Family Size and Birth Order' _Amer J
Epidemology_ 150:9:957-62 ('99)

is fairly convincing in claiming that the widespread decrease in male
births may be due to a biological preference for same sex in subsequent
births, coupled with the large decline in family size.  The Danish data set
studied is 1.4 million births, 1960-94:  While the ratio for 1st births
was: 51.2% male, by the time mothers with 3 previous boys gave birth, 52.4%
were males (the trend for demale births isn't discussed).  The increase
varied directly w/ # of of prior male births (p= 0.0007).

The authors don't speculate on what biologic mechanism may be at work; to
me it seems distinctly possible it is hormonal (possibly related to the sea
of hormones that can partially change the genentic sex of twins in the
womb?).  Rather, the authors say their data argue against chromoson toxins
and other env. influence hypothesis.  I can see that anthropgenic hormone
disruptors could play a role, but it does seem a huge influence that
declining family size is wiping out this heterogencity male birth advantage
(which seems due to their having a greater share of births to begin with,
thus less births erode the male heterogenity more than the presumed (no
data shown) female heterogenity, so the m:f birth ratio swings closer to
50%.).  Authors discuss Norwegian corroborating data.  Complicating factors
are social--families tend to keep having births when the previous ones are
of the same sex, and ifferent cultures may prefer males more than other
cultures.  This and another study rule out the 'parent's age difference'
hypothesis.

The well-known increase in male births during wars (that I mentioned in my
recent post on this subject, citing another piece of research) is discussed
briefly here, with nutritional, coital frequency, and "psychosocial"
hypothesis mentioned.

Tony Tweedale

Causality is a concept not subject to empirical demonstration. -David Hume
(1711-'76)

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