Studies in the social and biomedical sciences of racial differences in socioeconomic status or health within a population view the race of members as fixed and look for a difference in the frequency of a trait like average income or disease risk between racial subgroups. But, as I explain in this paper, there are good reasons to allow the race of members to vary with the trait whose variation within the population is to be described or explained. According to such (...) a view of race, racial categories are more scientifically significant if membership in the categories is allowed to vary with differences in scientific interest rather than held constant across a variety of interests. (shrink)