Common Sense. You wouldn't think it, but it's so incredibly UNCOMMON these days, it's hard to fathom how we got here! If you've ever wondered why common sense is lacking in so many people, it's directly due to developmental arrest.
One who is underdeveloped emotionally, never thinks Big Picture. They have no capacity for circumspection (being able to see a situation from multiple angles and perspectives). They lack empathy~ so whatever's happening, it's impossible for them to even consider YOUR point of view (or anyone else's)~ and they're emotionally reactive~ which puts em at a disadvantage, in context of thinking thru a problem or issue.
You wouldn't expect a 3 or 4 year old to have common sense. They haven't lived nearly long enough to have acquired the ability to think rationally or globally about various issues. Well, ya can't expect someone who's emotionally undercooked and living in an adult-sized body to, either. They are literally young children in big bodies.
One with BPD traits has little or no common sense. It's part of why we see them impulsively making poor choices and decisions that put them in harm's way.
Self sabotage is a common, everyday occurrence for one who is developmentally arrested. That old expression, "he couldn't find his way out of a paper bag" comes to mind here.
This is NOT to say, one with personality disorder traits isn't BRIGHT. Some I've worked with therapeutically have been exceedingly intelligent~ but common sense eludes em. For one who functions in a helping/healing modality of work, encountering this in a client or patient can present a special kind of frustration.
You just wanna shake these people sometimes, till their teeth rattle~ because what seems (to you) a painfully OBVIOUS solution to their immediate problem, is completely lost on them. It doesn't even OCCUR to them to think about it~ and there you are, trying to control your temper, while having to once again remind yourself, you're doing child psychology with a client who in reality, is under the age of 4.