I think Lee Lovet is a quack. We stutter in social situations because those situations are higher stress /more demanding. To use a real world example, let’s say you inherit a car. It runs ok for neighborhood driving but the second you take it on the highway it shakes, sputters, and breaks down forcing you to go slower, pull over, and go back to the neighborhood. Our speech systems break down under strain/stress. Stuttering is variable because life has varying demands