We learn from the passage that scientists ……..

Stuttering is the term given to the condition in which the sufferer speaks with difficulty because he or she cannot easily say the first sound of a word. Overall, there are about 50 million stutters in the world. Despite decades of research, the cause of stuttering is not known, though - contrary to popular opinion - it is not thought to be caused by emotional distress. Some believe it might be caused genetically, but scientists have been unable to pinpoint the actual reasons. What is known, however, is that it affects four times more men than women, and that 25% of all children go through a stage of development during which they stutter. Stuttering can be extremely demoralising. Those who are severely affected often attempt to avoid speaking situations altogether.