Research, not just gossip...
Research conclusively indicates that babies who sign tend to have a stronger command of verbal language and often begin speaking at an earlier age than babies who do not sign. Countless parents and caregivers have confirmed these findings with their personal experiences and observations. In addition, many Speech-Language professionals, pediatricians, and educators are supporting the use of signs to encourage early language development.

Daniels, M. (1996). Seeing language: The effect over time of sign language on vocabulary development in early childhood education. Child Study Journal, 26, 193-208.

Daniels, M. (2001). Dancing with Words: Signing for Hearing Children's Literacy. Westport, Connecticut: Bergin and Garvey.