To examine the relationship between v and population size further, we made use of a theoretical relationship between v and the scaled selection coefficient c (see [70, 71] ), which holds if nonsynonymous substitutions have equal (and small) selection coefficients, if synonymous substitutions are neutral, and if population sizes are sufficiently large (Methods). This relationship allows ratios of population sizes to be estimated from ratios of v estimates, under the assumption of constant selection coefficients across species. Here we further assumed that the ancestral population sizes of humans and the chimpanzee subspecies Pan troglodytes versus (to which the sequenced animal belonged) were roughly the same (N h = N c ) [5] , and estimated the ratio of v m in macaque to v h in human/chimpanzee from our 10,980 humanchimpanzee-macaque ortholog trios. Our estimate of v m / v h = 0.732 implies an estimate for the ratio of the macaque to human ancestral population sizes of N m / N h = 1.41 [bootstrapping 95% CI (1.15, 1.64)]. In comparison, the ancestral macaque population size has been estimated at ,73,000 [72] and ancestral human and chimpanzee population sizes at 40,000-70,000 [73, 74] , which would imply a ratio of 1.04-1.82, in reasonable agreement with our estimate. We used the same theoretical relationship to devise a LRT indicating whether or not each gene deviated significantly from the assumed model with N m / N h = 1.41 (Methods). For the vast majority (96%) of the 10,980 genes examined, no significant deviation was observed, indicating that the differences in selection pressure in macaque and the hominids are generally well-explained by differences in population size.