Inversion of subject NP and Aux in children's questions

Bellugi has proposed a stage of language acquisition in which children invert subject NP and Aux in yes-no but not wh questions. It was proposed that this stage would be found in a sampling of questions from young children. Cross-sectional samples of questions were collected from young children between 2:0 and 3:11. The results did not bear out this expectation. Based on percentages of inversion, none showed this stage. The results are discussed in terms of the importance of frequency in determining stages of acquisition and the use of children's data as psychological evidence for linguistic rules.