So maybe I am generalizing a bit, but I'm trying to get at something that isn't easily explained by science: why do so many psychiatrist families go bad in the same way?

In my experience (see, there's my disclaimer) psychiatrist-parents go wrong in a very specific way. They judge behavior, not the person.  It sounds like a good thing, I know.  For kids, it's a disaster.

Psychiatrists identify the behavior, but then focus on changing not the behavior directly, but the underlying cause of the behavior-- which is still not something intrinsic to the person.  If a guy with bipolar spends $10,000 in a week, psychiatrists link the behavior to the bipolar, and then try to medicate the bipolar.  (NB: "the patient has bipolar," not "the patient is bipolar.")

The obvious problem here is that maybe the guy spent $10,000 in a week because he doesn't give a damn? Or he wanted to impress some girl? i.e. just because someone has bipolar, doesn't mean every breath he takes is related to bipolar.  Psychiatrists are going to deny that they make it so simple, but in actuality they do: the moment you raise the dose of Depakote, you are sending the message that the behavior was related to bipolar. 

The psychiatrists with children-patients handle their kids in the same way.  They teach them what they are allowed to do and what they are not, what is acceptable and what is not-- but make no judgment on the kids themselves. Doing this denies the kid's identity, which is the whole purpose of childhood to begin with.  Rules then exist in an invented framework, or worse, in a vacuum.  There's no internalization of the rules; there's no superego.  Just some arbitrary limits on id.

If you tell a kid that a behavior is unacceptable, the kid has learned nothing about himself; he's only learned that this one thing is something he can't do. But if you make the kid own it-- make the behavior part of his identity, then he has a chance to change his identity.  Instead of learning it is unacceptable to take his brother's potato chips away, he can learn that he has a choice: to be the kind of person who takes chips, or the kind of person who doesn't. 

I understand the trickiness of this; you don't want to make the kid feel like he is a bad person.  But you do have to find a way to teach him that if he does that thing again and again, then he is a bad person.  Is that what he wants?  Who are you, kid?  Who do you want to be?  This also allows his to take personal credit for doing something good:

And you can see the creation of a future borderline here.  For God's sake, will someone please tell me who I am?  Give this storm of emotions some context?  Right now, I get angry/sad/thrilled/terrified over nothing, it just comes over me-- I wish I could be angry/sad/thrilled/terrified over something.  But all people ever do is tell me what I can and can't do.  If I do something bad, people freak. If I do something good, no one even notices.  No one likes me for me, they just over/underreact to what I do.

There's a second lurking trouble: parents' control of their affect. 

The psychiatrist isn't supposed to get mad at his patient; but then he comes home, and tries very hard not to get mad at his kid-- just tells him the behavior is unacceptable, gives him a time out, whatever.  But guess what?  The psychiatrist is exhausted, eventually his patience runs out, and BAM! a tsunami of anger.

calm&firm...calm&firm...calm&firm...EXPLOSION.

The explosion part can come at any time, depending on how much patience the parent has that day.  And that's exactly the problem.  What does the kid learn? That this ethereal rulebook for what is acceptable and unacceptable only has two, binary results: no affect, or all affect-- and you never know what you're going to get.

In the biz, this is called inconsistent parenting.

What the kid needs to know are the rules of the game; they need the parent to be consistent, predictable, so that they can be safely chaotic, experimental, exploratory off of your foundation.   You want her to know exactly how you'll react if she tries pot, you want a superego so well constructed you're superfluous.   And you want levels of emotion, different things get you more or less angry.  We know you went berserk because your boss is a big jerk whose been riding you all day, but your two year old thinks you went berserk because she spilled the milk.  Geez, sorry.  May as well try heroin, what's the difference?

It doesn't necessarily mean you have to be an angry parent-- your predicted reaction could be anxious acceptance or loving disappointment-- but it has to be predictable.  And it has to be about the kid, not the behavior.  The kid needs to know you're connecting with them, not what they do, or else they'll think that the only way to connect is by behaviors.

You can see the further development of a borderline here: what the hell do I have to do to get some emotional response from you?  Kill myself?  Keep pushing until you finally blow up?  I don't even feel like I'm alive, but I'm not sure that you are either-- or is it just me, that I matter so little that I can't even get a little affect? You're insanely jealous if I talk to another guy, but you totally ignore me when I'm with you. At least with jealousy you're being real with me. Etc.

Trust me on this: at age 2, a kid feels your rage and your love the same.  It's exciting, and they haven't yet learned to fully differentiate the two feelings.  What counts is the amount of emotion, not which emotion.  (Horror movies and porn are the same to a 14 yo for this reason.)  Fast forward 20 years-- that all-out screaming match with your boyfriend felt weirdly relaxing.

And so you have a scenario: busy psychiatrist, often tired, can't generate much emotion past anger-- and it can come at any time.  No deep connection with the child as a person-- as their kid, yes; as the sum total of their behaviors, yes-- but not as a developing individual.  The kid learns that as long as some things are done correctly-- e.g. school-- they can get away with other things that the parent won't notice, e.g. pot.

Oh, and this is the best part: if the kid (adult or child) becomes a psychiatric patient, they now have a bond with their parent-- and the parent's over-involvement in their kid's psychiatric care is the framework for a relationship.  It's analogous to helping them build a go cart. 

And that's all the kid ever wanted anyway.