Common sense wisdom also suggests "spare the rod, spoil the child". The research has no way of taking this type of long term effect into account, so they always "discover" whatever they wanted to discover.
For example, water can be harmful to children. But they need it to survive just like everyone. Same with discipline. They are not untouchable or little angels all of the time. Think about what message you're sending to the kid by not disciplining them early: "do whatever you want, the world will never lay a finger on you"? Why would you lie to your kid like that, give them totally false expectations?
So I think it's not such a black and white thing as a lot of these studies are trying to paint it to be.
It is black and white. Hitting someone without their consent is a violation of the nonaggression axiom. It violates the most fundamental right of self-ownership. They're sentient individuals, not their parents' property.
@cogliostro I'm not talking about "one right way to raise a child," I'm talking about striking others without their consent. Yes, it is wrong for everyone. If someone "cannot be reached or taught" then just accept that peacefully. Your lessons can't possibly be so wonderful that beating them into people can be justified.
Whose nonaggression axiom? Let's not be so foolish and arrogant as to presume that there is only ONE CORRECT way to raise a child, or that the same approach is even going to work on every single person.
Just as there are some adults in the world who understand only the language of violence, so too there are children who cannot be reached and taught to behave without a controlled dose of violence.
Spare the rod is more accurately translated to "shepherd's hook" meaning an unguided, not beaten, sheep will stray from the flock.