Put simply, gender meant the social and psychological characteristics of a woman or a man. Gender was built on the foundations of "sex" - those biological characteristics of men and women based upon differences in their reproductive roles. The key point was that gender was to be separated from sex. It was intended to be an explanation for the fact that, although women and men might always be identified by biology, at different historical moments and in different cultures their social characteristics varied - what they wore, how they behaved, their location within the division of labour, their sexuality and so on.
One history of gender sees its origins in the work of Margaret Mead, whose anthropological work was published in the 1930s. She memorably told us about assertive, practical, unadorned Tchambuli women and their skittish, gossipy, decorated men. Mead's work has now been discredited but part of her legacy was gender - a term for the inconstancy of the human sexes.
What rapidly became attached to the idea of gender was a feminist concern to use it to analyse discriminatory social practices. Not just "difference", then, but inequality. Girls were not poor at mathematics because they suffered the malign influence of a uterus, as a popular 19th-century notion had it, but because of social practices working against them in the school. Difference and inequality fused so that inequality came to be seen as the origin and result of difference. And this did not mean calling for an end to vive la difference since who knew what men and women would be like if they lived in a new world where the men did not exploit the women? Certainly not Macho Men nor Earth Mothers.
Gender did what all useful scientific concepts do: it provided a framework within which researchers could operate. What were the practices that operated against women and girls? How did they work? The result was a rich stream of writing that transformed academic and popular understanding. Gender changed the world.
But recent studies have disturbed these ways of thinking. The first unsettling development for the sex/ gender distinction has come from social historians who tell us that sex is far from constant. The barber-surgeons of the Renaissance performed their operations in theatres before an audience. Their anatomists observed cadavers on the slab and carefully drew them but, remarkably, what they saw was not two sexes but one. Where we see difference everywhere, from the cut of a jaw to the curve of an eyebrow, they saw similarity and variation around a one-sex model. They imagined the vagina as an interior penis, the labia as foreskin, the uterus as scrotum and the ovaries as testicles. And it is important to understand that this model was not superseded simply by advances in our understanding of human bodies. In fact many subsequent medical discoveries, such as those in developmental embryology, fit very well into a one-sex model. Knowledge of the common form of the male and female foetus in the womb came at a time when everyone had given up looking for similarities and, like good gender theorists, looked only for difference.
So the mother who asks after the "gender" of her new-born baby is demonstrating that sex, and sexing, is a social practice with its own history. Sex is gender too.
John Hood-Williams is a contributor to the `Sociological Review'