Interesting point. I never thought about it that way.
I have to confess that I used the “if you don’t behave you might be forced by the courts to go live with your father” line once or twice. Talk about putting the fear of God into the kid - I only did that a few times before it became unnecessary. She had visited in their house overnight twice, and that was enough to know she did not want to be there full time.
Diabolically clever. You evoke the fear of the father's way of life, along with the ham-handed stupidity of the courts, all in one little phrase.
I remember a woman with 10 kids saying “My husband wouldn’t cheat on me because I’ll divorce him and give him custody of the kids.”