When I was a kid, I didn't stutter at all when I talked to myself while I was alone. However, the old I get, I notice now I stutter even when I'm talking to myself. But, I blame that on my stuttering getting worse as I get older due to increased anxiety, no matter what the situation. It's interesting that a lot of us have/had the same experience not stuttering when alone, but I think the person who commented about our threshold has a great point.