Humans in many societies are known to mate, or marry, assortatively for a number of characters such as eye colour, height, IQ and place of birth. In this assortment an element of active choice may be involved. It is not known whether this choice is genetic. Two models of human mate choice are examined in which both males and females can express a mating preference. In the first, 'sexual' preferences can be expressed for any phenotype not necessarily one's own; in the second, preferences are o...