Not entirely sure of the researched answer on that one, but I do know two little girls who were both taught signs, and everytime the mums used sign they said the words they wanted the girls to say. One little one started putting words together & talking well around 20 months and the other around 23 months. Both have really good manners at eating time & saying what they want or do not want. So... in watching them grow up with using some signing, I've decided to do the same... I think their speech development was "normal" but I'm sure someone might have a more technical answer LOL