Can the sex of the second child be predicted by the birth-weight of the first child?

Abstract 

The reproduction costs to a human mother are different if she has a son as opposed to a daughter. According to the Trivers–Willard hypothesis, evolution should promote those females who, having the ability to invest in the more expensive sex, are also able to adjust the sex of their offspring accordingly. It is therefore possible that a mother's biological condition (which is also connected with her reproductive potential), as measured by the neonatal weight of her first child, can be a good predictor of the second child's sex. From data for 227 healthy mothers from Wroclaw (Poland) we show that the probability of giving birth to a boy in the second pregnancy is higher after a relatively heavier first-born child (ANOVA, F(1,225)=3.79, P<0.053). This relationship, however, is only significant after a first-born daughter (F(1,117)=9.66, P<0.002) and not after a first-born boy. Some possible explanations of the fact that only the birth-weight of a first-born daughter – and not a son – can be a good predictor of the secondary sex ratio are also discussed.

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• f1 Correspondence to: Boguslaw Pawlowski, Department of Anthropology, University of Wroclaw, ul. Kuźnicza 35, 50-138 Wroclaw, Poland. Phone: +48 71 3752 774; Fax: +48 71 3752 697; E-mail: bogus@antropo.uni.wroc.pl