There is a good 8 part doc on Youtube, from Louis Theroux who spent time with the Phelps:
Some of them, the women, are quite highly educated. Shirley? is a practising lawyer and some of the other women are either studying or practising law.
But they are all in total thrall to the patriarch, Fred, who is indeed quite nasty.
Louis shows at one point how the small children, the next generation, are taught to hate, it doesn't come naturally, it has to be taught.
You can see how even with good intelligence, Shirley? and her kin become the way they are and I think, gain enjoyment and self-esteem from their behaviour.