    Sentence 1: Well-known theories of child  developmentFreud’s. Skinner's. Gesell's.  and Piagets. for example‘provide litte  comfort since dramatc shifts in. favored  theories have occurred ‘since the  launching of systematic study. of children  about 100 years ago.    Sentence 2: Freud. Skinner. Gesell and  Piagetall studied child development