To investigate the question of whether baby signs programs help children acquire language earlier or advance their cognitive development, in 2003 researchers J. C. Johnston and colleagues examined seventeen recent studies of baby signing. Johnston found that the studies did not support the big claims made by the infancy industry, that baby signs help make babies smarter or acquire other languages sooner. Many of the studies they scrutinized only looked at a few children, had poor follow-up (they didn’t look at their language development for long enough periods of time), and little detail (for example, not enough information on how often the parents and children signed), and overall, the industry’s claims went much too far. What’s more, the researchers noted that “parents can be stressed by the challenges of meeting demands of work, caring for a young child, and other family and personal obligations, and experience guilt if they feel they are not doing everything recommended by infancy specialists and the infancy industry.”
Source: King, K., & Mackey, A. (2007). The bilingual edge: Why, when, and how to teach your child a second language. New York: Collins
 

Online English lessons for kids: dinolingo.com