There are several factors that could be involved, such as biological differences. People with bipolar disorder appear to have physical changes in their brains. The significance of these changes is still uncertain but could eventually help in one way or another to identify the causes. There are also neurotransmitters whose imbalances in natural brain chemicals, called neurotransmitters, appear to play an important role in bipolar disorder and other mood disorders. Bipolar disorder is more common in people who have a first-degree relative, such as a sibling or parent, who has the condition. Researchers are trying to find genes that could be involved in bipolar