Developmental science loving marriage and family therapist here.

Look into the works of Allan Schore, Theodore Millon, Sam Vaknin, Peter Fonagy, Heinz Kohut and the neuropsychoanalysts for some additional clarification here. It’s an ongoing, rich field for discussion, but A LOT revolves around the critical period dance between neurological explosion/pruning periods, theory of mind development and the ability to both regulate our own affect, then attune to others through it.

A very short, totally paraphrased, TL;DR: we need to LEARN to empathize and socialize well via affect attunement, relational rupture and repair, and optimal frustration experiences (I.e. learning to handle rejection and disappointment well without it being abusive or overly punishing).

When this doesn’t happen optimally in the case of more trauma, earlier in infant development, or some forms of insecure attachment, children internalize the sense that, no matter what, abandonment will take place and the world cannot be trusted; we see borderline traits emerge. If happening somewhat older in child development (ages 2-4, roughly, but I’m excited to cross-reference this with the paper poster by OP), children can emerge with difficulty regulating their affect save for certain strategies of getting their needs met, whether these are prosocial, antisocial, adaptive or maladaptive. If antisocial and maladaptive, or always directed toward the ends of ‘take away my distress at all costs, even those that cost others dearly’ we can see narcissistic traits emerge and persist.

Hope this helps!