During the nineteenth century, occupational information about women that was provided by the United States census-a population count conducted each decade-became more detailed and precise in response to social changes. Through 1840, simple enumeration by household mirrored a home-based agricultural economy and hierarchical social order: the head of the household (presumed male or absent) was specified by name, whereas other household members were only indicated by the total number of persons counted in various categories, including occupational categories. Like farms, most enterprises were family-run, so that the census measured economic activity as an attribute of the entire household, rather than of individuals.

The 1850 census, partly responding to antislavery and women's rights movements, initiated the collection of specific information about each individual in a household. Not until 1870 was occupational information analyzed by gender: the census superintendent reported 1.8 million women employed outside the home in "gainful and reputable occupations". In addition, he arbitrarily attributed to each family one woman "keeping house". Overlap between the two groups was not calculated until 1890, when the rapid entry of women into the paid labor force and social issues arising from industrialization were causing women's advocates and women statisticians to press for more thorough and accurate accounting of women's occupations and wages.

    Anthropologists once thought that the ancestors of modern humans began to walk upright because it freed their hands to use stone tools ,which they had begum to make as the species evolved a brain of increased size and mental capacity. But discoveries of the three-million-year-old fossilized remains of our hominid ancestor Australopithecus have yielded substantial anatomical evidence that upright walking Appeared prior to the dramatic enlargement of the brain and the development of stone tools. Walking on two legs in an upright posture (bipedal locomotion) is a less efficient proposition than walking on all fours (quadrupedal locomotion) because several muscle groups that the quadruped uses for propulsion must instead to provide the biped the biped with stability and control. The shape and configuration of various bones must likewise be modified to allow the muscles to perform these functions in upright walking. Reconstruction of the pelvis (hipbones) and femur (thighbone) of "Lucy", a three-million-year-old skeleton that is the most complete fossilized skeleton from the australopithecine era, has shown that they are much more like he corresponding bones of the modern human than like those of the most closely related living primate, the quadrupedal chimpanzee. Lucy's wide, shallow pelvis is actually better suited to bipedal walking than is the rounder, bowl-like pelvis of the modern human, which evolved to form the larger birth canal needed to accommodate the head of a largebrained human infant .By contrast, the bead of Lucy's baby could have been no larger than that of a baby chimpanzee.

    If the small-brained australopithecines were not toolmakers, what evolutionary advantage did they gain by walking upright? One theory is that bipedality evolved in conjunction with the nuclear family: monogamous parents cooperating to care for their offspring. Walking upright permitted the father to use his hands to gather food and carry it to his mate from a distance, allowing the mother to devote more time and energy to nurturing and protecting their children. According to this view, the transition to bipedal walking may have occurred as long as ten million years ago, at the time of the earliest hominids, making it a crucial initiating event in human evolution.