Human Reproduction, Vol. 15, No. 6, 1422,
June 2000
© 2000 European Society of Human Reproduction and Embryology
Letters to the Editor |
The association between the sexes of children within sibships
The Galton Laboratory, Department of Biology, University College London, Wolfson House, 4 Stephenson Way, London NW1 2HE, UK
Dear Sir,
Jacobsen et al. (1999) analysed the sexes of 614 000 children born in Denmark between 1980 and 1993. These authors wrote: ‘We have observed no biologically or statistically significant association between the sex of a child and ... the sexes of the preceding children in the family (Lexis variation)’. This sample of births has recently been expanded to 1.4 million in Denmark between 1960 and 1994 (Biggar et al., 1999
).
These authors reported that the probability that a current birth is male, is directly related to its number of prior brothers, and inversely related to its number of prior sisters. The trend is reported as highly significant (P = 0.0007), and these authors cite similar data in respect of Norwegian births between 1950 and 1985. Similar data have also been offered in respect of 4 million French births (James, 1975), and a large sample of USA White births (Ben-Porath and Welch, 1976).
Thus the phenomenon seems general. In principal there are two possible explanations viz: (i) that P (the probability of a male infant) varies across sibships (Lexis variation) and/or (ii) that P varies positively with (and as a consequence of ) the sex of the previous sib(s) (Markov variation).
I know no persuasive evidence for Markov variation in human beings: moreover Astolfi and Tentoni (1995) failed to find significant evidence for Markov variation in the sexes of the offspring of 266 000 cows. Accordingly it seems reasonable to conclude that there is Lexis variation in human beings viz that couples vary in their probability of producing a boy. Work is in progress to estimate this variation.
References
Astolfi, P. and Tentoni, S. (1995). Sources of variation of the cattle secondary sex ratio. Genet. Select. Evol., 27, 3–14
Ben-Porath,Y. and Welch, F. (1976). Do sex preferences really matter? Q. J. Econ., 90, 285–307
Biggar, R.J., Wohlfhart, J., Westergaard, T. and Melbye, M. (1999). Sex ratios, family size and birth order. Am. J. Epidemiol., 150, 957–962
Jacobsen, R., Møller, H. and Engholm, G. (1999). Fertility rates in Denmark in relation to the sexes of the preceding children in the family. Hum. Reprod., 14, 1127–1130
James, W.H. (1975). Sex ratio and the sex composition of the existing sibs. Ann. Hum. Genet., 38, 371–378[Web of Science][Medline]
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