In the early and mid-20th centuries this was a form of bowdlerisation, concerned with some of the more violent elements of nursery rhymes and led to the formation of organisations like the British 'Society for Nursery Rhyme Reform'.
Psychoanalysts such as Bruno Bettelheim strongly criticized this revisionism, on the grounds that it weakened their usefulness to both children and adults as ways of symbolically resolving issues and it has been argued that revised versions may not perform the functions of catharsis for children, or allow them to imaginatively deal with violence and danger.
The lyrics were first printed in close to their modern form in the mid-eighteenth century and became popular, particularly in Britain and the United States in the nineteenth century.
From this period we sometimes know the origins and authors of rhymes—for instance, in "Twinkle Twinkle Little Star" which combines the melody of an 18th-century French tune "Ah vous dirai-je, Maman" with a 19th-century English poem by Jane Taylor entitled "The Star" used as lyrics.
Early folk song collectors also often collected (what are now known as) nursery rhymes, including in Scotland Sir Walter Scott and in Germany Clemens Brentano and Achim von Arnim in Des Knaben Wunderhorn (1806–1808).
The first, and possibly the most important academic collection to focus in this area was James Orchard Halliwell's, The Nursery Rhymes of England (1842) and Popular Rhymes and Tales in 1849, in which he divided rhymes into antiquities (historical), fireside stories, game-rhymes, alphabet-rhymes, riddles, nature-rhymes, places and families, proverbs, superstitions, customs, and nursery songs (lullabies).
By the time of Sabine Baring-Gould's A Book of Nursery Songs (1895), folklore was an academic study, full of comments and foot-notes.
The definitive study of English rhymes remains the work of Iona and Peter Opie.