Approximately 2-4% of genetic material in human populations outside Africa is 
derived from Neanderthals who interbred with anatomically modern humans. Recent 
studies have shown that this Neanderthal DNA is depleted around functional 
genomic regions; this has been suggested to be a consequence of harmful 
epistatic interactions between human and Neanderthal alleles. However, using 
published estimates of Neanderthal inbreeding and the distribution of mutational 
fitness effects, we infer that Neanderthals had at least 40% lower fitness than 
humans on average; this increased load predicts the reduction in Neanderthal 
introgression around genes without the need to invoke epistasis. We also predict 
a residual Neanderthal mutational load in non-Africans, leading to a fitness 
reduction of at least 0.5%. This effect of Neanderthal admixture has been left 
out of previous debate on mutation load differences between Africans and 
non-Africans. We also show that if many deleterious mutations are recessive, the 
Neanderthal admixture fraction could increase over time due to the protective 
effect of Neanderthal haplotypes against deleterious alleles that arose recently 
in the human population. This might partially explain why so many organisms 
retain gene flow from other species and appear to derive adaptive benefits from 
introgression.