It is incorrect to believe that tickling a baby will cause stuttering. I have read this several times and have marveled at how people can believe it. For good information about stuttering, people should go to http://www.stutteringhelp.org/. There you can find pages of information including Facts on Stuttering that states "What causes stuttering? There are four factors most likely to contribute to the development of stuttering: genetics ( approximately 60% of those who stutter have a family member who does also); child development (children with other speech and language problems or developmental delays are more likely to stutter); neurophysiology ( recent research has shown that people who stutter process speech and language in different areas of the brain than those who do not stutter); and family dynamics ( high expectations and fast-paced lifestyles can contribute to stuttering).
Stuttering may occur when a combination of factors comes together and may have different causes in different people. It is probable that what causes stuttering differs from what makes it continue or get worse."
Maybe this comment was originally started by a mother or grandmother who was trying to get a sibling to stop tickling a baby. I don't know of anyone who really enjoys being tickled, at least not for any length of time.