Anyone can develop bipolar disorder, but it is most common among people between the ages of 18 and 35. About 1 in a 100 people develop bipolar disorder at some stage during their lives, and it is as common in men as in women.
The exact cause of why some people develop bipolar disorder is not known. However, there are several important factors which are known to play a part in causing the illness. Bipolar disorder can be inherited; people whose parents have bipolar disorder are also more likely to have the illness (although in the majority of cases, children of parents with bipolar disorder do not develop the illness).
People with bipolar disorder have been found to have a disturbance of the chemicals in the brain, which is why the illness can often be controlled with medication.
Stressful life events may increase the chance of developing bipolar disorder among those who are at risk (e.g. those with a family history of the illness) and stress can also make further episodes of either mania or depression more likely in those who already have developed the disorder