Dr Marilyn Daniels found that preschoolers who were taught sign language scored significantly higher on the Peabody Vocabulary Test when compared to preschoolers who did not learn sign language. Daniels concluded that a preschooler's vocabulary can be improved if words are presented visually and kinesthetically as well as verbally.
Daniels, Marilyn, Dancing with Words: Signing for Hearing Children's Literacy. Bergin & Garvey, October 2000. ISBN: 0897897927.