They theorised that there are distinct stages to how children develop and apply rules to negatives and questions.
The basic syntax of a question: Where (question word) is (auxiliary verb) Daddy (object) going (main verb)
Stages of Question Formation
1. Use of intonation to signal a a question is being asked (remember Cruttenden with his children recognising intonation) .
2. The use of question words. (What, where, why)
3. Manipulating syntax to create more detailed questions?
Stages of Negative Formation
1. The use of the negative alone (such as the infamous NO! or NOT! Stage)
2. Combining a negative with others in the 2-word and telegraphic stages, usually at the beginning. (Not me, no want, no beddy)
3. Using the negative in the middle of the utterance (I not want that, Me no like that)
4. Increasing accuracy of negative words and the use of contractions with auxiliary (She isn't going, I don't want to)
5. Increased complexity and range of negative words. (I haven't got any. There isn't anything. I have not got my juice.)
6. Saying no without using negatives (inflection and adjusting intonation).