Agent-Based Modelling shows that Operation of Fisher's Principle does not explain Zero Heritability in Human Offspring Sex Ratio

The ratio of male to female births has garnered interest and debate for more than a century. According to Fisher's principle, offspring sex ratio remains approximately equal through a process of negative frequency-dependent balancing selection. That is, individuals whose alleles predispose them to produce the rarer sex will have greater fitness, causing these alleles to increase in frequency until the population sex ratio is even again and the allele is no longer advantageous. Recently, though, analysis of all Swedish births since 1932 revealed that offspring sex ratio is not heritable, demonstrating that Fisher's principle cannot operate in humans. Some have argued that Fisher's principle makes no prediction about the heritability of offspring sex ratio at equilibrium and therefore can still serve as a valid explanatory framework. We used agent-based modelling to simulate the operation of Fisher's principle and its effects on genetic variation in offspring sex ratio over 5000 generations. Our simulations demonstrate that the operation of Fisher's principle at equilibrium does not actively eliminate heritability, contrary to implicit assumptions in previous arguments. These results, combined with recent evidence that offspring sex ratio is not heritable in humans, suggest that Fisher's principle is not a valid explanatory framework for human offspring sex ratio. ### Competing Interest Statement The authors have declared no competing interest.