This finding fits my view on stuttering. Genetics drives the onset of stuttering in many people. The other factors (alone or in combination with genetics) are internal developmental issues caused by unique environmental events like neurological incident, virus and head trauma. Only some of these events were reported in the twin study, so the effort is measured as moderate. Needless to say that, as in many other conditions, the social environment plays a negligible role in the actual onset.
Here is the abstract:
Heritability and environmental effects for self-reported periods with stuttering: A twin study from Denmark.National Centre for Epidemiology, Surveillance and Health Promotion, National Institute of Health, Rome, Italy.
AbstractAbstract Genetic influence for stuttering was studied based on adult self-reporting. Using nation-wide questionnaire answers from 33,317 Danish twins, a univariate biometric analysis based on the liability threshold model was performed in order to estimate the heritability of stuttering. The self-reported incidences for stuttering were from less than 4% for females to near 9% for males. Both probandwise concordance rate and tetrachoric correlation were substantially higher for monozygotic compared to dizygotic pairs, indicating substantial genetic influence on individual liability. Univariate biometric analyses showed that additive genetic and unique environmental factors best explained the observed concordance patterns. Heritability estimates for males/females were 0.84/0.81. Moderate unique environmental effects were also found.