The Galton Laboratory, University College London, Wolfson House, 4 Stephenson Way, London NW1 2HE, UK
Zorn (2004) and Bisioli (2004
) discussed the nature of explanation appropriate to the established rise in sex ratio (proportion male) at birth during and just after the two World Wars. Bisioli (2004
) contrasted proximate causes with ultimate causes: he noted that from the latter point of view the increases in sex ratio are an adaptive response to the decimation of males. However, as pointed out by Zorn (2004
), the notion of ultimate causes seems not to be helpful here because the increases in sex ratios stopped soon after hostilities had ceased, and before the numbers of males had been restored. So is there any evidence that, as I had proposed (James, 1995
), human sex ratios at birth are under some form of stabilizing homeostatic control? I tried to test this notion empirically by examining whether sex ratios at birth responded to secular movements of sex ratios of the breeding populations in a number of countries in the past 50 years (James, 2000
). I found little direct evidence to support this hypothesis. So we may provisionally conclude that, at present, the notion of the proximate cause is the more useful one. If there are adaptive mechanisms at work (as evolutionary biologists would expect), we may infer that some form of constraint operates to preclude their manifestation.
Meanwhile, it is worth noting that proximate causes can account for the established wartime variations of sex ratios at birth as follows (James, 2003):
(i) The rises during and just after World Wars I and II were ex hypothesi occasioned by higher coital rates during wartime leaves and demobilization leaves. The generality of these rises was questioned by Zorn et al. (2003). However, a reading of Russell (1936
) leaves the matter in no doubt in regard to World War I: Graffelman and Hoekstra (2000
) (who were cited approvingly by Zorn et al., 2003
) used a statistical sledgehammer to crack a nut that had been opened 64 years previously.
(ii) The decline in Iran during the IranIraq War (Ansari-Lari and Saadat, 2002) may have been partially due to the effects of mustard gas. It may also have been due to psychological stress.
(iii) The decline associated with the 10 day War in Slovenia, 1991 (Zorn et al., 2002) was also presumably due to psychological stress. It is noteworthy that this war was not associated with short wartime and demobilization leaves (and the high coital rates typical of young, reunited couples): this circumstance may be construed as evidence in favour of the suggestion that the high sex ratios typical of belligerent countries during and after prolonged wars are indeed due to high coital rates.
References
Ansari-Lari M and Saadat M (2002) Changing sex ratio in Iran 19762000. J. Epidemiol. Community Health 56,622623.
Bisioli C (2004) Sex ratio of births conceived during wartime. Hum Reprod 19,218219.
Graffelman J and Hoekstra RF (2000) A statistical analysis of the effects of warfare on the human secondary sex ratio. Hum Biol 72,433446.[Medline]
James WH (1995) What stabilizes the sex ratio? Ann Hum Genet 59,243249.[Medline]
James WH (2000) Secular movements in sex ratios of adults and of births in populations during the past half-century. Hum Reprod 15,11781183.
James WH (2003) Sex ratios of births conceived during wartime. Hum Reprod 18,11331134.
Russell WT (1936) Statistical study of the sex ratio at birth. J Hyg (Lond) 36,381401.
Zorn B (2004) Reply: sex ratio of births conceived during wartime. Hum Reprod 19,219220.
Zorn B, Sucur V, Stare J and Meden-Vrtovec H (2002) Decline in sex ratio at birth after 10-day war in Slovenia. Hum Reprod 17,31733177.
Zorn B, Sucur V, Stare J and Meden-Vrtovec H (2003) Untitled reply. Hum Reprod 18,11341135.