A study published in the journal Science analyzed genetic data and found that interbreeding between Neanderthals and anatomically modern humans (AMH) occurred predominantly between Neanderthal males and AMH females[1][2][3]. A 62% relative excess of ancestral AMH compared to autosomes has been reported on Neanderthal X chromosomes[1][2]. This pattern is 1.6 times higher on the X chromosomes of a Neanderthal woman from Siberia and similar sites dated at 80,000 and 52,000 years ago[3]. The lack of Neanderthal DNA on the X chromosomes of modern humans supports a sex bias in admixture[2][3]. Modeling has shown that partner preference is a more likely cause than purely demographic processes with differential migration of men and women[2]. The authors did not reject the role of demographic factors or negative selection on Neanderthal variants[1][2][3]. Interbreeding is estimated around 250,000 years ago[3].