1 The Galton Laboratory, University College London Wolfson House, 4 Stephenson Way London NW1 2HE, UK
Dear Sir,
Orvos et al., reporting on small samples of births write: `Our results suggest that the secondary sex ratio is influenced by the maternal age and maternal parity' (Orvos et al., 2001). Literally dozens of papers have investigated the influence of these variables (and paternal age) on sex ratio. In general, results on small samples have been inconclusive and contradictory. Since there are no prior hypotheses, it seems reasonable to pool samplesso data have been published on large national samples. In reviewing these, I suggested that the only conclusion to be drawn was that any variation of sex ratio with these variables is both tiny and, possibly, labile with time (James, 1987
). It seems worth repeating a sentence from that paper: `Almost all of the variation in sex ratio (with maternal age, parity and paternal age) discussed above falls between the values of 0.516 and 0.513: apparently these variables are not `close' to the causes of variation of sex ratio, and so are unlikely to yield any useful clues to those causes.'
Obstetrician-gynaecologists looking for such clues may well find them in the established very substantial variation of offspring sex ratio with various forms of placental pathology. Highly significant male excesses are associated with abruptio placenta, placenta praevia, fatty liver of pregnancy and toxaemia; highly significant female excesses are associated with placenta accreta and extrauterine pregnancy (James, 1995). I have adduced very large quantities of data to support the hypothesis that offspring sex ratio is causally associated with parental (including maternal) hormone concentrations around the time of conception (James, 1996
). So it is natural to wonder whether the same hormone profiles that are responsible for one sex or the other are also responsible for placental pathologies. Whether this is true or not, it is clear that placental pathologies provide more fruitful clues to the causes of variation of sex ratio than do maternal age, paternal age or parity. This is simply because the variation of sex ratio is so much greater with the former than with the latter variables.
References
James, W.H. (1987) The human sex ratio. Part 1: a review of the literature. Hum. Biol., 59, 721752.[ISI][Medline]
James, W.H. (1995) Sex ratios of offspring and the causes of placental pathology. Hum. Reprod., 10, 14031406.[Abstract]
James, W.H. (1996) Evidence that mammalian sex ratios at birth are partially controlled by parental hormone levels at the time of conception. J. Theor. Biol., 180, 271286.[ISI][Medline]
Orvos, H., Kozinsky, Z. and Bartfai, G. (2001) Natural variation in the human sex ratio. Hum. Reprod., 16, 803.
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