In a sample comprising 4 753 269 offspring of 3 543 243 Swedes, we detected no significant genetic influence on offspring sex ratio. In fact, our heritability estimate was zero, with an upper 95% CI of 0.0020 (i.e. two-tenths of one per cent), rendering Fisher's principle untenable as a framework for understanding human offspring sex ratio.

Our results also rule out the possibility that offspring sex ratios are adaptively calibrated to individuals' heritable traits. Certain interpretations of the Trivers–Willard effect [6] propose that parents who possess any heritable trait that disproportionately benefits the fitness of one sex will bias their offspring sex ratio towards that sex [8]. A number of studies have reported evidence of such effects––e.g. male-biased offspring sex ratio in bigger, taller [8], wealthier [11], higher status [7] and less sociosexually restricted [10] parents, and a female-biased offspring sex ratio in more physically attractive parents [9]. Several of these findings, though, have been questioned on statistical grounds [33–35], and other studies have not supported the hypothesis (including very large birth cohort studies, e.g. [8,36,37]). Our findings are incompatible with the basic effect: if offspring sex ratio was calibrated to heritable traits, then it would necessarily be heritable to some degree as well.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7062014/