I really liked learning about this. I couldn't find the original article I read yesterday but will post this one instead:

Narcissists are so cruel partly because of something called '''object constancy''' - Business Insider

This really resonated with me because when my partner and I get into an argument, he resorts to name-calling and "break-up" threats. I used to wonder why he was reacting to such a degree that he'd cry, storm off, hide at his parents, drive off.. after discussing a seemingly minor concern. I then felt he just didn't want to be with me anymore and was looking for an excuse to get out of this relationship.

He claims he does NOT want us to break up but is so reactive. So this piece made total sense to me.

The other article pointed out 3 characteristics of NDP (with a pretty good description):

1) hypersensitive
2) low to no empathy
3) no object constancy (although I think it read "object permanence" instead)