In about 50% cases of PDS, genetic connection is found. Is it true? For the rest, is it safe to assume that their stammering is the outcome of environmental factors (early negative speech experiences and learned response to that)? Thanks in advance!This is a common misconception, even among most SLTs. If 50% is genetic, then the rest is non-genetic. Most geneticists call non-genetic environmental, but this is highly misleading for the non-expert mind. A better wording is to talk about developmental factors and socio-environmental factors. Developmental is mainly the process by which the fertilized egg is developing to an adult human being. The key phase is up to age 5. Developmental issues can arise due to allergy, malnutrition, random incidents like hitting your head, and illnesses like virus infections. In the strict sense they are of course environmentally driven, but it's more about a healthy environment needed by the body to build itself. What most people mean by environment is the social environment of the child. So it's wrong to say that 50% is due to social environment. This number is very likely low, maybe 10-20%. And 30%-40% is due to developmental issues.
That is often what the blank slaters (i.e. we are born equal) think. X (intelligence, personality, stuttering) might be 50% genetic, but we are so eager to point out that 50% is environmental, i.e. societal. But of course, they got it completely wrong. Most of the environment consists of neurobiological development. So the society, e.g. parents, schools, and social interactions, is at best 10%-20% in many cases.