Due to the very nature of bipolar disorder, it is often not diagnosed until adulthood, often in the late-20s to early-30s. Typical markers begin to manifest themselves in late adolescence and early adulthood, but diagnosis can take a decade or more. It’s not a single illness, but rather a category of disorders and, because it is episodic by nature, it takes many, many years for a solid diagnosis to emerge.
The NIMH believes that there is no single cause of bipolar disorder and there remains no proven organic cause. There is sometimes a genetic predisposition, as it can run in a family, but it is just as often unrelated to genetics. There is no single genetic marker, either, which makes finding the link difficult.
As for Britney, there is no possible way to tell from her public outbursts exactly what is going on. She doesn’t seem to be exhibiting any of the tell-tale signs of classic biploar disorder, but has obviously had some sort of mental breakdown. She looks to be having a sort of psychotic break or a series of them – disorganized thought, socially odd behavior and impairment being signs – and psychosis is an extreme edge of the manic phases, but it is much more likely that it is a function of her insane stress level or severe clinical depression than bipolar disorder.