Speech-Language Pathology student here. At least in the early period around 3 years of age, the proportion of cases where stuttering appears but then wanes out in about 6-12 months is higher in girls than in boys. There seems to be a genetic/hormonal link to that, where testosterone hinders the development of the left hemisphere in the brain, which is responsible for speech and language functions in the wast majority of people. This is one of the current widely accepted theories. This also holds truth for other developmental speech and/or language disorders. Hope my answer helped!