Assortative mating is a nonrandom mating pattern where individuals with similar genotypes and/or phenotypes mate with one another more frequently than what would be expected under a random mating pattern. For example, it is common for individuals of similar body size to mate with one another. Less commonly, in negative assortative mating, individuals with diverse traits mate more frequently than what would be expected in random mating. Both cases cause the frequency of certain genotypes to differ greatly from the frequencies predicted by the Hardy-Weinberg Principle, which states that allele and genotype frequencies should remain constant under a random mating system. Assortative mating does not change the frequency of individual alleles, but increases the proportion of homozygous individuals. By contrast, disassortative mating results in a greater number of heterozygotes.