Sex is a universal and ancient basis for social differentiation. All societies divide work and status along gender lines. The biological realities of the sexes have lead to social practices of men as protectors/breadwinners and women and procreators and care givers. Sex distinctions varies from one culture to another, man can also be caregivers, with low fertility rate women can be breadwinners, because the female distinction fades. Societies differ and ways and extent to which they dramatize sex differences, some enlarge them, some diminish. Societies vary in degree to which they make sex differences seem large or small, important or unimportant. The way societies rationalize these distinctions also differ, some refer to religion, other draw on secular practices of equality, while others refer to pseudo-scientific theories of sex differences. Study of gender relations was not present in sociology until the mid-20th century.