Variation of human sex ratios at birth by the sex combinations of the existing sibs, and by reproductive stopping rules: comments on Garenne (2009).

Garenne (2009) presented data on the sex ratio of a present birth by the numbers of previous brothers and sisters. In unisexual sibships, the probability of a further girl increases with the number of previous girls; and the probability of a further boy increases with the number of previous boys. Garenne noted that there is an asymmetry in that the effect is stronger with regard to girls than boys. He was uncertain of the cause of this. Here I suggest a potential solution to this problem. Garenne also seems to imply that parental reproductive stopping rules cause heterogeneity of sex ratios. I suggest that they may reveal it - but do not cause it. Moreover, I suggest that the effects of such stopping rules may be counter-intuitive.