Positive assortative mating occurs more frequently than negative assortative mating. For animals, mating strategies include random mating, disassortative mating, assortative mating, or a mating pool. Sexual preference creates a tendency towards assortative mating or homogamy. Another form of sociological assortative mating is assortative mating based on racial and ethnic background. Genetic assortative mating is well studied and documented. With complete monogamy, there is assortative mating for sexually selected traits. Divergence is frequently distinguished by assortative mating between individuals of the two species. Statistical research looking at eleven disorders found widespread assortative mating between people with mental illness. Koinophilia differs from the "like prefers like" mating pattern of assortative mating. This assimilation reduces the prevalence of this form of assortative mating. The patterns of racialized "assortative mating" in Brazil is complex. The second common type of assortative mating occurs with respect to coloration. Different factors may apply simultaneously to result in assortative mating in any given species. This initially suggested the possibility of assortative mating by female choice. Several hypotheses have been proposed to explain the phenomenon of assortative mating.