Nursery rhyme is used for traditional poems and songs for young children in Britain and many other countries. However, most nursery rhymes were not written down until the 18th century. Nursery rhymes seem to have come from a variety of sources, including traditional riddles, proverbs, ballads, lines of Mummers' plays, drinking songs, historical events, and it has been suggested, ancient pagan rituals. Many nursery rhymes have been argued to have hidden meanings and origins. For example, Katherine Elwes' book, The Real Personages of Mother Goose (1930), in which she linked famous nursery-rhyme characters with real people. She assumed that children's songs were a peculiar form of coded historical narrative, propaganda or covert protest, and rarely considered that they could have been written simply for entertainment. There have been several attempts, across the world, to revise nursery rhymes. In the late 19th century, the major concern seems to have been violence and crime, which led leading children's publishers in the United States to “improve” mother goose rhymes. In the 20th century, revisionism of nursery rhymes became associated with the idea of political correctness. Most attempts to reform nursery rhymes on this basis appear to be either very small scale, light-hearted updating, or satires written as if from the point of view of political correctness in order to condemn reform. The controversy was over changing the language of Baa Baa Black Sheep in Britain from 1986. It was alleged, uncorroborated assumption that it was a complaint against Medieval English taxes on wool, and a historic narrative about the slave trade. However, some psychoanalysts strongly criticized this revisionism, on the grounds that revised versions might not function as ways of symbolically solving issues for children and adults or imaginatively dealing with violence and danger.