This keeps reproducing inequality through the system of social stratification. Economic inequality seems to be related to the influence of inherited social-economic stratification. Four principles are posited to underlie social stratification. Annette Lareau also addresses the factors that lead to social stratification in educational attainment. Greater degrees of social stratification appeared in agrarian societies. There are different models that attempt to describe the relationship between gender and stratification. The Hopewell inherited from their Adena forebears an incipient social stratification. An internal wave is a gravity wave that moves along density stratification within the ocean. Southern migrants were often treated in accordance with pre-existing racial stratification. This is interpreted as evidence of increased social stratification. In spite of official socialist policies of equality, social divisions and social stratification remained. Such mobility is sometimes used to classify different systems of social stratification. By doing so the seed is undergoing a natural form of "cold stratification" or pretreatment. Culturally, the Sassanids implemented a system of social stratification. Even the indirect reference to types implicit in the notion of stratification can be eliminated.