Despite a revived interest in explaining the evolution of anisogamy in recent years (i.e. different—micro and macrogametes), there remain more questions than answers. The topic is important because it is thought to be the foundation of the theory of gender differences and relations. Twelve of these questions are briefly reviewed here—(1) the distinction between sex and sexual types; (2) the distinction between mating types and anisogamy; (3) the possible role of ecological as well as social evolution in proto-gender differences and (...) relations; (4) the life history strategies involved; (5) whether the relevant social relationships are based on conflict or cooperation; (6) the origin and properties of meiosis; (7) the conformity of theories with sex allocation theory; (8) the relevance of multiple levels of selection; (9) appropriate modelling strategies; (10) the relationship between the kind of gametes produced and secondary sexual characteristics; (11) the relevance to humans; and (12) how realistic the search for a single explanation is. It is concluded that polarized sexuality may be a form of trade. (shrink)