 Primarily for this reason, many population-genetic models assume that Hardy-Weinberg equilibrium obtains; this is tantamount to assuming that mating is random with respect to genotype. But is this assumption empirically plausible? The answer is sometimes but not always. In the human population, for example, mating is close to random with respect to ABO blood group, so the genotype that determines blood group is found in approximately Hardy-Weinberg proportions in many populations (Hartl and Clark 2006). But mating is not random with respect to height; on the contrary, people tend to choose mates similar in height to themselves. So if we consider a genotype that influences height, mating will not be random with respect to this genotype (see section 3.4 ‘Non-Random Mating’).