Agreed, I did a quick scan and could only find this. Still almost two decades old. “In older children and adults the male-to-female ratio is large, about 4 to 1 or greater (Craig, et al., 2002)” The 4 to 1 ratio is the most common accepted ratio in the stuttering, therapy, and research communities. My assumption is there is really no need to do a new study as the ratio may be frequently confirmed when researchers do other studies that incorporate both sexes.