1. Brown, R. (1968). The development of wh questions in child speech. Journal of Verbal Learning and Verbal Behavior, 7, 277–290.
2. Cohen, M.A., & Gross, P.J. (1979). The developmental resource: Behavioral sequences for assessment and program planning. New York: Grune & Stratton.
3. Collins-Ahlgren, M. (1975). Language development of two deaf children. American Annals of the Deaf, 120, 524–539.
4. Dennis, W. (1960). Causes of retardation among institutional children: Iran. Journal of Genetic Psychology. 96, 47–59.
5. Dennis, W., & Dennis, M.G. (1937). Behavioral development in the first year as shown by forty biographies. Psychological Record, 1, 349–361.