Sex-ratio biasing towards daughters among lower-ranking co-wives in Rwanda.

There is considerable debate as to whether human females bias the sex ratio of their offspring as a function of their own condition. We apply the Trivers-Willard prediction-that mothers in poor condition will overproduce daughters-to a novel measure of condition, namely wife rank within a polygynous marriage. Using a large-scale sample of over 95 000 Rwandan mothers, we show that lower-ranking polygynous wives do indeed have significantly more daughters than higher-ranking polygynous wives and monogamously married women. This effect remains when controlling for potential confounds such as maternal age. We discuss these results in reference to previous work on sex-ratio adjustment in humans.