I have had many discussion with others off these boards about NPD and MLC. It seems that some personality disorders are learned responses. MLCers certainly exhibit the signs of NPD and other non-empathetic behaviours, and my therapist, having met my h, thought he had a borderline personality disorder.
FWIW I think they hold it together for a long time. Snodderley talks about 'masks' and [looking back over my marriage] that this was certainly correct for my h. The borderline personality traits, carefully held in check for 10, 20, 30 even 40 years, seem to take over. Others have called it Jekyll and Hyde, where Mr Hyde gets into the driving seat
