It's never one thing though. A parent that is abusive is likely lacking in a lot of other areas which can further exacerbate whatever becomes of the child. Stress, low intelligence, alcoholism, neglect, low income, overworked, divorce, etc etc all go hand in hand with an abusive parent or parents.
You can be a parent that never raises a hand towards your child, but if you work 14 hours a day and barely attend to their basic needs, they are going to be equally as fucked.
Personally, I don't have a problem with occasionally swatting a 3 or 4 year old to correct them. But you can't tell average people that because some are stupid and lazy and will take it to the extreme.