Gerald Maguire has stuttered since childhood, but you might not guess it from talking to him. For the past 25 years, Maguire — a psychiatrist at the University of California, Riverside — has been treating his disorder with antipsychotic medications not officially approved for the condition. Only with careful attention might you discern his occasional stumble on multisyllabic words like “statistically” and “pharmaceutical.” Maguire has plenty of company: More than 70 million people worldwide, including about 3 million Americans, stutter — that is, they have difficulty with the starting and timing of speech, resulting in halting and repetition.