Or do they know exactly how to hit below the belt? I love intj's, I suspect my Dad is one, I know a few others, but my God, they know exactly what to say that will inflict the most pain. If I had to define it by type, they are the only ones I feel threatened by most but at the same time I ♥ them the most out of other types.
There is a time when the same sex parent becomes the preferred object of torture for a teenager. That was certainly true in my case. My older INTJ son would torture me to try to get what he wanted. If he succeeded, then he would stop--he was persistent, practical, and goal oriented. When I complained to my INFJ wife, she would warn me that if I thought my INTJ son was bad, wait until the younger INFJ son became a teenager. He was a master at twisting the knife and he did it somewhat, I think, to practice his INFJ skills! I have survived my sons' teen age years intact, the wounds have healed, and both my sons are nice, charming, and successful young men, whose skills were well honed on my poor, wounded psyche. Bringing up kids is not easy. But, I don't think that INFJ's realize the power they are capable of wielding, particularly when they are young and somewhat awkward in its application.