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Siberian live birth sex ratios and the SPrOO hypothesis
  1. P H Jongbloet
  1. Department of Epidemiology and Biostatistics, University Medical Centre Nijmegen, PO Box 9101, 6500 HB Nijmegen, Netherlands; p.jongbloetepib.umcn.nl

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    Melnikov and Grech found a highly significant seasonal pattern of the sex ratio (SR) at birth in western Siberia, namely, a peak in the second and a trough in the fourth quarter of the year.1 This peak and trough are in line with the seasonal preovulatory overripeness ovopathy (SPrOO) hypothesis, which states (1) an approach to gender equity at the peak of the “ovulatory” seasons, (2) preferential fertilisation of non-optimally matured oocytes by Y-bearing sperm during the transitional stages between them, and (3) SR reversal because of excess of male biased fetal loss during the “anovulatory” seasons.2 The mentioned peak would correspond, just like in non-human mammals with the breakdown of the “ovulatory” season in spring; the trough with SR reversal during the “anovulatory” season in winter.

    The authors wonder about the annual secular trend in a fall and then rise of the SR with the turning point in early 1980s, being different from the continuous decline in industrial countries over the past half century. We have argued that the rise in SRs before the first world war in Finland3 and in many other countries concurred with continuous improvement in living conditions, education, and reproductive hygiene, and, thus, a decrease of conceptopathology rate, and in turn, increase of male surviving fetuses.4 The fall of the SRs after the turning point around the second world war was interpreted as consequence of further amelioration of the ovulatory and conception pattern, reflected by the concurrent decrease in pregnancy wastage. The same reasoning accounts for the initial very low and then increasing and again decreasing SRs in developmental countries that are in demographic transition going hand in hand with amelioration of socioeconomic conditions. This may be compared with the rise and fall in socially upward family conditions.5 The odds for delivering a male child increases (while pregnancy loss diminishes) when the socioeconomic level increases from low to moderate up to a plateau and then decreases (despite continuation of decreasing pregnancy wastage) when this level increases further from moderate to higher.

    The relatively low SRs in western Siberia, when compared with other countries in west Europe, may also be related to higher rates of conceptopathology because of the extreme climatic variations and inherent stronger seasonal variation in reproduction further away from the equator. This suggests higher rates of male biased pregnancy loss as the underlying mechanism. The SR increase in the early 1980s would mean that this rate is diminishing and that further progress in socioeconomic conditions will result in still higher SRs and ultimately in a decrease, in analogy with those in west European countries.

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