I'm not certain if bipolar disorder is over diagnosed, but my sense is that in people diagnosed it may be. I put it that way, because I think there are still many people out there with bipolar disorder (of some type) that have yet to even get/go to a psychiatrist.

I think your points about some people getting BP dxs instead of BPD are valid, and I agree that Borderline Personality Disorder does have significant features that shouldn't make it that difficult to differentiate from bipolar disorder. Your point about a psychiatrist really taking their time to differentiate between the two (and asking the right questions) is indeed crucial. I sometimes feel that a small number of patients themselves "feed" the information they want to their psychiatrists. That means maybe leaving out details, overemphasizing symptoms they read in the symptom list for the mental illness they self diagnose themselves with, and similar tactics. Of course perhaps the psychiatrist improperly leads them into that, too.

The whole issue of emotional mood lability and ultra or ultra rapid cycling likely causes misdiagnosis, too. It could go either way depending on the psychiatrist's view on ultra or ultra rapid cycling.

I know that people with bipolar disorder do have mood lability, but more "mood" lability than "emotional". I also think that medications could play some part. It always surprises me how often antidepressants are used for people with bipolar disorder. Not that they can't be helpful in conjunction with a moodstabilizer and/or antipsychotic, but really, wouldn't an antidepressant alone NOT exacerbate the issues of Borderline PD alone? Maybe it wouldn't help much (they say DBT is more effective), but shouldn't hurt as much as antidepressant monotherapy for bipolar disorder.

Another issue about "over diagnosed" bipolar disorder separate from Borderline Personality Disorder is possible bipolar diagnosis in people who actually have unipolar depression. I believe the opposite was mostly true in the past, but nowadays I have heard of some psychiatrists being too "bipolar happy", especially now when there is a bipolar spectrum concept. Are some people given Cyclothymia or even bipolar type 2 diagnoses incorrectly? Have some of these folks ever really experienced hypomania? People without a mental illness can have periods of feeling damned good or extremely angry. Or even acting a little wild. I think a good psychiatrist has to be very careful to decide what falls into the hypomania range and what was/is in some other range. That is likely hard sometimes and may take time. Full mania observed is far easier to accurately label.