Bipolar disorder (& other brain disorders) are complicated. It's not just one chemical. It's a complex system of various parts of the brain not operating as they should do. Also, stability in the childhood home plays a part. If a person inherits the brain disorder but grows up in a stable home, they will get the symptoms but be better able to deal with those. Here's a study I found: Brain Explorer - Focus on Brain Disorders - Bipolar Disorder - Aetiology