The structure, function, and even the definition of the family have varied tremendously from culture to culture, and for different social groups within each culture. They have changed over time because of internal developments or contacts with other cultures. Not all families centred on a sexual relationship, but most did, institutionalized as marriage, though in this there was wide variety as well. Norms and patterns of sexual familial relationships were how groups defined themselves, maintained their distinctions from other groups, and reinforced hierarchies within the group. Since the nineteenth century, scholars have developed theories of family and kinship, initially seeing evolutionary stages but now emphasizing variety and divergent lines of development, using qualitative and quantitative sources. They have still found major points of transition in family life: the foraging families of the Paleolithic became sedentary crop-raisers, with intensified social hierarchies; centralized states attempted to control reproduction through laws and norms governing marriage and sexual relations; patterns in family life became more rigid in classical cultures and text-based religions; colonialism and industrialization slowly altered family life and norms of sexuality; government intervention in family life expanded in the twentieth century. Today there is an increasing diversity of family forms around the world.