Sex ratio allows for an interesting classroom exercise in Ascertainment Bias. The sex-ratio of Fisher's direct progeny was about as skewed as our lambs last year - 2M:6F; except in the other direction. We'd have been delighted to get 75% ewe-lambs. Many people associate Fisher with sex-ratio because he articulated (he was neither the first, nor the last, nor the most clear) the idea that sex-ratio naturally equilibrates at 1M:1F. His argument ran thus:
- If males are less common than females, a random male is more likely to have offspring than a random female; some of whom will be left "on the shelf" in a monogamous society
- Any parents which then acquire a mutation that makes them have sons will have more grandchildren
- Therefore these male-bearing genes will spread through the population . . .
- . . . until males are more common than females or until their minority-report advantage dies away as sex-ratio approaches equality
- Same reasoning applies if you start with females being less common.
Actually, it turns out that sex-ratio at birth is not quite 1:1 - in all societies where these things are recorded (and that is all societies which can count up to two), there is an excess of boys at birth: something around 105M:100F. Boys are fragile in the neonatal period and their mortality is higher than their sisters. Statistics on still-births and from abortions show that this post-natal mortality among boys is a continuation of a trend in utero with the primary sex ratio (at conception) estimated at 130M:100F. That 130 figure is widely cited but it's quite hard to find the data on which this fact is based. Later on, in many societies including Ireland, the sex-ratio takes another nose dive as boys start to drive cars (far too fast); commit suicide, which may be the same thing; and do silly dangerous things to impress girls and prove that they are well 'ard. By some age between 25 and 40 the sex-ratio is really 1:1. Thereafter women consistently and increasingly outnumber men of the same age: there are between 5x and 9x more female centenarians.
clouded by pipe-smoke [R] when he dissed Richard Doll's statistical analysis of association between smoking and cancer.
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