My little cousin can use about 10-15 different signs but has yet to speak one single word.
Do you think it's simply because the visual part of the brain develops faster than the audible part? Or that it's just an easier part of the brain to use?
Do you think that their brain just doesn't know how to tell their mouth to physically form the word, or that they just can't THINK of the word that they want?
Have you known a baby who uses sign language before speaking?