Furthermore, females who were able to become pregnant were supposed to be kept pregnant, so they could make more slaves to sell. The differences in skin color found in the United States make it obvious how often black women were impregnated by whites. For example, in the 1850 Census, 75.4% of "free negros" in Florida were described as mulattos, of mixed race. Nevertheless, it is only very recently, with DNA studies, that any sort of reliable number can be provided, and the research has only begun. Light-skinned girls, who contrasted with the darker field workers, were preferred.