It is most definitely overdiagnosed in the US. Mainly for insurance reasons and many of those diagnosed are only diagnosed that way so the insurance covers them and they dont actually have bipolar. They tend to do this a LOT for personality disorders as they are not covered by insurance to my knowledge.

I wrote a thread on here months back about the over diagnosis of BP in the US, it was a great discussion, search for overdiagnosis in the US and you might be able to find it.