Was Interbreeding between Neanderthals and modern humans strongly sex biased?

Platt et al. (Science 391, 922–925, 2026) report that excess anatomically modern human (AMH) ancestry on Neanderthal X chromosomes, and the reciprocal depletion of Neanderthal ancestry on modern human X chromosomes, constitute evidence for predominantly male-Neanderthal × female-AMH mating. I show that both signals fall within neutral drift nulls under symmetric mating. Using forward Wright-Fisher simulations in a declining Neanderthal population, 22.8% of symmetric-mating replicates produce an X:autosome AMH-ancestry ratio equal to or exceeding the reported Altai value of 1.62. Under a realistic out-of-Africa bottleneck trajectory, 28.1% of symmetric-mating replicates produce an X:autosome Neanderthal ancestry ratio equal to or below the observed AMH depletion of ~0.6 (Skov et al. 2020). Additionally, the three Neanderthal genomes used as converging evidence share a single ancestral introgression event (~250,000 years ago) and do not constitute independent observations. Sex-biased mating remains a plausible hypothesis, but the reported X:autosome signals are insufficient to distinguish mating asymmetry from drift. This repository contains the full manuscript, simulation code (R), and both figures.