According to The Stuttering Foundation, "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."
ASHA has this on their web site: "The exact cause of stuttering is unknown. Recent studies suggest that genetics plays a role in the disorder. It is thought that many, if not most, individuals who stutter inherit traits that put them at risk to develop stuttering. The exact nature of these traits is presently unclear. Whatever the traits are, they obviously impair the individual's ability to string together the various muscle movements that are necessary to produce sentences fluently.
Not everyone who is predisposed to stutter will develop the disorder. For many, certain life events are thought to "trigger" fluency difficulty. One of the triggers for developmental stuttering may be the development of grammar skills. Between the ages of 2 and 5 years, children learn many of the grammatical rules of language. These rules allow children to change immature messages ("Mommy candy") into longer sentences that require coordination to produce fluently ("Mommy put the candy in my backpack"). A child who is predisposed to stutter may have no difficulty speaking fluently when sentences are only one or two words long. However, when the child starts trying to produce longer, more complex sentences, he or she may find himself or herself not quite up to the challenge—and disfluent speech results.
After stuttering has started, other factors may cause more disfluencies. For example, a child who is easily frustrated may be more likely to tighten or tense speech muscles when disfluencies occur. Such tension may increase how long a disfluency lasts. Listeners' responses to stuttering (e.g., teasing) can aggravate fluency difficulties as well. People who stutter vary widely in how they react to the disfluencies in their speech. Some appear to be minimally concerned. Others—especially those who have encountered unfavorable reactions from listeners—may develop emotional responses to stuttering that hinder speech production further. Examples of these emotions include shame, embarrassment, and anxiety."
In some cases, knowing the cause or what makes stuttering worse, will enable parents of a child who stutters to change things at home so the stuttering lessens or will enable the PWS to be able to adapt his/her lifestyle to lessen the stuttering. Avoiding stress or learning to cope with it may help. For those of us who have a genetic factor for the reason we have stuttering in our family, we need to accept it and learn the best ways to deal with it.