When is it appropriate to intervene in parents' childrearing practices?

There are many places where one can find out research about best parenting practices. Yet parents often disagree with those practices, or just do things the old-fashioned way, perhaps because they were raised that way, and it didn’t appear to hurt them. Spanking is one example of this kind of divide, but I’m sure there are others.

I guess this is a cost-benefit kind of question. Suppose it becomes very clear that disciplining children can be done most effectively without physical discipline of any kind. But spanking isn’t all that bad. Many of us were spanked, and it didn’t scar us forever. Besides which, sometimes our children just aggravate us so, we can’t help it. They learn from it. So, doesn’t that justify it? It’s merely a difference in parenting philosophy, and we’re all correct in how we choose to parent, so long as we don’t go across the line into abuse.

Or, does a community have a right to intervene with parents, and to educate them about best practices, and to do their best to stop parents from physically or emotionally hurting their children? Under what circumstances does a community have this right? Does it have to be only in cases where the life of the child is endangered, or are there other cases where it would be appropriate?