Many factors may combine to cause bipolar disorder. No one knows for sure, but because it is a disorder that runs in families, in some people it is likely to be caused by the genes they inherit. It is rather like diabetes or hair colour in this respect. In other people there is no family history (although this does not mean it is not genetic).

The intermittent nature of the disorder strongly suggests that its origin is chemical or biochemical. Episodes of mania and depression may be associated with changes in neurotransmitters, the substances that carry messages in the brain. For some people these may be more prone to change in times of stress or at different times of year. Some research has indicated a significant seasonal variation in the prevalence of mania and depression. It is also possible that the use of recreational drugs may trigger episodes in some individuals.