Are there any studies on what causes people to perpetrate extreme cases of child abuse, such as repeated beatings, chaining children to beds and starving them (among other horrible things), etc. Things where the perpetrator(s) have to know what they're doing, have to think about it, where there has to be intent and not short-term impulse. Why do they do it? What causes someone to do it? How do they justify it to themselves initially and as it goes on, before any bullshit they make up to justify or minimize it to themselves or others? What do we know about the causes of these cases that isn't also true in far more cases in which someone doesn't do such things even when they have the opportunity on multiple occasions?

Please please please provide sources, preferably ones that aren't pop psychology articles.

Edit: Since apparently this wasn't clear enough the first several times I have asked this, I'm going to put it in big letters so it's impossible to miss.

Cite your sources!