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National Stuttering Association

This is a great question! This is also an important question, because until we fully understand what causes stuttering, we can only treat the symptoms of stuttering. As the question implies, there is an undeniable genetic component to stuttering. If you do stutter, you’re about three times more likely to have a relative who stutters (Andrews, 1983). We also know that stuttering is not entirely genetic. Several people have been identified in the world that stutter, but have an identical twin who does not; if stuttering were entirely genetic, this would be impossible. Causation can be broken down into 3 categories:

Predisposing factors – What makes people susceptible to stuttering?
Precipitating factors – What makes people begin to stutter?
Maintaining factors – What makes people continue to stutter?

For this article, I want to talk primarily about predisposing factors. Research is teaching us more and more about stuttering. Brain scans are beginning to shed some light on how the brains of people who stutter (PWS) differ from brains of people who don’t stutter; for example, while speech and language are processed almost exclusively in the left hemisphere for the vast majority of the population (somewhere around 95%), studies are showing that brains of PWS are far more likely to have speech and language processed in the right hemisphere, or distributed more evenly between the 2 hemispheres. Alas, this is not the case for all stuttering brains; and, we can’t say whether the unusual wiring in some brains causes stuttering, or if stuttering leads to atypical brain wiring. If we abandon physiology for the moment, I have a behavioral model which I use sometimes to help students and parents better understand how stuttering may come about (and why it stays with some people and not others). Imagine that everyone is born with a certain measurable predisposition (a ‘dosage’ of sorts) for stuttering; if it runs in your family, you are likely to be born with a larger ‘dose’ than most. Now imagine that this dosage is like a stick, stuck in the bottom of a pond. If the stick is long enough, or of the water is shallow enough, we can see the stick. The length of the stick will never change, but the water level in the pond certainly can. Now imagine that the water level represents the ‘threshold’ for stuttering, and when the stick is visible, stuttering is present. The stick (stuttering) may be barely perceptible, or it may be hard to miss. The water level (threshold) can rise or fall, due to any number of factors; physiological, psychological, or environmental. Several observations make this dosage/threshold model for stuttering development compelling:

  • Speech is both a language and a motor task. As language and motor skills are rapidly developing in young children, the threshold will be lowered, and indeed many young children begin to stutter. We know that girls mature more quickly, and indeed more girls ‘grow out’ of stuttering than boys. If girls do have a higher ‘threshold’, this may help explain why they are less likely to have all speech and language disruptions than boys.
  • For many PWS, stuttering goes up as fatigue sets in. If the threshold is variable, then surely fatigue would compromise it. Stuttering can vary not only throughout the day, but also from week to week, or month to month. Physiological changes could impinge on the threshold; many PWS stutter more when they are in poorer health.
  • If there is such a threshold, it could also be tied to fear and anxiety, which we know correlate highly with increases in stuttering. It is also worth noting that emotions are predominantly processed in the right hemisphere.
  • Girls who stutter have more stuttering relatives than boys who stutter (Andrews & Harris, 1964). Likewise, girls who stutter should on average have a higher predisposition (‘dose’) of stuttering than boys who stutter. This would be consistent with the belief that girls have a higher threshold than boys (see #1). Remember that I have not addressed precipitating factors, which trigger the onset of stuttering, or maintaining factors, which allow stuttering to continue and progress. All three types of factors work together to result in chronic developmental stuttering.

Charley Adams, PhD, CCC-SLP