Not true!
This was a compelling theory in the 1920s - again from the United States. It lasted for decades and some children went through periods with one hand tied behind their backs in an attempt to "undo the damage". Eventually the theory was properly tested, and there was no evidence to support it - lots of children had been made to use their right hands without "knock-on" effects at all. Left-handedness doesn't seem to be relevant.
However, there has been considerable interest over the years in the possibility that there may be neurological explanation for stammering, namely that it might be linked with which side of the brain is responsible for organising the skills of speech and language.
Scientific advances, using brain scanning techniques, are beginning to offer us a greater understanding about the way in which the brain is organised for speech and language. Recent research has investigated possible differences in cerebral dominance (being "right- or left-brained"!) between adults who stammer and those who don't. Early results appear to be showing that there may be subtle variations which might impact upon speaking skills.
However it is important not to jump to conclusions because the experiments can only be carried out on adults at the moment. It is possible that these differences are a consequence of stammering, rather than being a cause of the problem.
