1. START EARLY. When they start flopping around when you are changing their diapers, smack them.
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Are you kidding?
No, really, are you kidding?
No. An 8 or 10 month old who repeatedly disobeys is already testing the boundaries. A smack on a bare bottom is not abuse. It's a reminder. Please notice that I didn't say "beat." I said "smack." It doesn't have to be hard, just hard enough to get their attention.