Modern anthropology developed at a time when a great many human societies were illiterate, and much of the research on incest taboos has taken place in societies without legal codes, and, therefore, without written laws concerning marriage and incest. Nevertheless, anthropologists have found that the institution of marriage, and rules concerning appropriate and inappropriate sexual behavior, exist in every society. The following excerpt from "Notes and Queries on Anthropology" (1951), a well-established field manual for ethnographic research, illustrates the scope of ethnographic investigation into the matter: