A longitudinal study funded by the National Institutes of Child Health and Human Development showed that babies who used symbolic gestures understood more words, had larger vocabularies, and engaged in more sophisticated play than non-signing babies. Parents of the signing babies in the study noted decreased frustration, increased communication, and enriched parent-infant bonding. Signing babies also displayed an increased interest in books.

The researchers revisited the families in the original study when the children were seven and eight years old. The children who signed as babies had a mean IQ of 114 compared to the non-signing control group's mean IQ of 102.

You can read about these findings in the following articles:

Acredolo, L.P., Goodwyn, S.W., Brown, Catherine, Impact of Symbolic Gesturing on Early Language Development - Journal of Nonverbal Behavior, 24, 81-103.

Acredolo, L. P., & Goodwyn, S.W. (July 2000). The long-term impact of symbolic gesturing during infancy on IQ at age 8. Paper presented at the meetings of the International Society for Infant Studies, Brighton, UK.

Another study by Acredolo and Goodwyn shows that signing babies engage in more joint attention episodes than their non-signing peers. Joint attention happens when a baby directs an adult's attention to something he or she is interested in. A child who is more skilled in initiating joint attention episodes can help accelerate his or her verbal language development. The findings in this study helps to explain why signing babies tend to start to talk earlier their non-signing peers.

Brie Moore, Linda Acredolo, & Susan Goodwyn (April 2001). Symbolic gesturing and joint attention: Partners in facilitating verbal development. Paper presented at the Biennial Meetings of the Society for Research in Child Development.