This chapter explores the evolutionary origins of stratifi cation and its neurohormonal underpinnings. The status striving that eventually leads to SES stratifi cation is a hard-wired behavior because high status equals increased fi tness. However, I want to allay concerns that some may have that all this talk of hard-wiring means that social class is fi xed (biologically inherited) across generations and that social structure does not matter. I show that social structure matters immensely because it determines the extent to which genes matter for the attainment of social status. Personal traits have a large impact on achieving social class level in some societies, but hardly at all in others. I also want to show that social class cannot be genetically fi xed by brief discussions of regression to the mean, the Hardy-Weinberg equilibrium, and decreased assortative mating, all of which contribute to mixing-up the classes in modern societies.