Two things provoked me this week to think about the costs of over-protective parents. First, there is this post by Daniel Reeves. It echoes a theme well-known to followers of Lenore Skenazy that parents over-estimate the probability of harm. It points out that this also happens to non-parents and when they interact -- say, because a lost child isn't found because someone didn't want to be accused of abduction -- the consequences can be tragic.
Second, there is this episode of This American Life. If you are a parent -- over-protective or not -- you will be surely driven to despair about the first story of a child who grew up effectively in restraints. Not physical ones but extreme boundaries.