No Evidence of Genetic Basis to Variation in Human Offspring Sex Ratio

Fisher's principle proposes that the population sex ratio remains approximately equal (around 1:1) through a process of negative frequency-dependent selection. However, large-scale studies show that human offspring sex ratio (OSR) is not heritable in humans, raising doubts about whether Fisher's principle is a valid explanatory framework for human OSR evolution. Song and Zhang recently proposed a mechanism where OSR propensity is genetically influenced but produces an OSR phenotype with negligible observable heritability due to probabilistic sex determination and small family sizes. Here, we conducted an evaluation of this proposal by simulating the proposed mechanism under a broader range of assumptions, revealing that it only produces undetectable OSR heritability under a specific set of parameters. We further explored whether Fisher's principle could operate under such low OSR heritability and found, by reanalysing simulation outcomes, that there is only a very narrow parameter space where Fisher's principle operated and OSR heritability was realistically low. We also make multiple corrections to Song and Zhang's original analyses which weaken their claim. In sum, our assessment is that evidence for a genetic mechanism that yields undetectable OSR heritability, but allows the operation of Fisher's Principle, is weak. The proposed mechanism lacks sufficient evidence to overturn the simpler conclusion that OSR is not heritable in humans, and consequently that Fisher's principle is not a valid explanatory framework for human OSR.