Is the Sign Advantage a Robust Phenomenon? From Gesture to Language in Two Modalities

Phenomena are robust to the extent that they remain invariant under a variety of conditions. An important task in studying any phenomenon is identifying its boundary conditions, that is, the most extreme conditions under which the phenomenon still holds. It has been claimed that a sign advantage phenomenon characterizes language acquisition, based on evidence that young children acquire manual signs at an earlier age than they acquire spoken words. The boundary conditions for this phenomenon were explored by using data from the Toddler Sign Program, a 9-month program of bimodal input and assessment that involved 25 handicapped and nonhandicapped toddlers, 11 to 33 months old at onset. Although the total input to each child strongly favored speech, a sign advantage was obtained in the two youngest children and in two speech-delayed children with Down's syndrome. However, loss of the sign advantage prior to syntax suggests that it is primarily a prelinguistic phenomenon. Finally, it was found that the manual modality is a highly robust vehicle for early vocabulary, whereas the speech modality shows variation according to handicap status.