People who outgrew or had their stuttering diminish does not refute genetics as an indicator. You may hear celebrities who "used to" stutter go on talk shows and speak just fine, but many have gone on record in interviews with stuttering groups or long form interviews to reveal that stuttering is still a big preoccupation for them, even if they don't experience it nearly as much as they used to. And for people who are mild stutterers who choose to be covert, they're just as much stutterers as the rest of us, you just dont see it because they hide it. But even beyond all that, stuttering has a genetic component. The evidence for it is basically conclusive: http://www.stutteringhelp.org/genetic-and-neurological-factors-stuttering. That doesn't mean environment doesn't play a role, but it if was mostly environment we'd have a lot more stutterers than 1% of the population. Also I want to suggest that we don't downvote questions like this. OP is curious about the issue and is asking a genuine question.