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Bipolar disorder, like all mood disorders, is still somewhat of a mystery to scientists because the study of the brain itself is still a young science.   Nobody knows what causes bipolar disorder.  One theory is that chemicals inside the "brain on bipolar" fluctuate abnormally.  A patient develops mania if too much of the chemical is present, and depression if the chemical levels drop too low.   It is clear, however, that bipolar disorder runs in families and has a strong genetic component.  Although people are at higher risk if a family member has been diagnosed, doctors believe that the disorder kicks in partly because of a stressful event, drug abuse, or other unknown factor.

Among identical twins, for example, one twin may develop the disorder while the other does not.  Since identical twins share all of the same genes, but are influenced by different environmental factors, this finding suggests that bipolar disorder is caused as much by the environment as it is by genes.  Indeed, most people with bipolar family members do not become ill themselves.  

Nonetheless, it is clear that bipolar disorder tends to run in families, because in comparison to the general population, children whose parents or siblings have bipolar disorder are four to six times more likely to develop the illness.  Researchers believe, therefore, that bipolar disorder has a genetic component and are looking for genes that increase a person's chance of developing the illness. Bipolar disorder is a chronic disease, equally common among males and females.  It usually appears between ages 15 and 30; only in rare cases do children or adults over 65 get newly diagnosed.  

And yet, even though bipolar disorder is less common among children than adults, a small number of children do have it, and their symptoms differ from those of adults.  For example, children with bipolar disorder cycle more rapidly between depressed and manic states, and they often experience more mixed states. Mania among children shows up as irritability and rage, symptoms that are similar to those of Attention Deficit Hyperactivity Disorder (ADHD).  For this reason, child bipolar disorder is often misdiagnosed.  

Older adults with bipolar disorder may have lived their entire lives without getting diagnosed, and when their symptoms finally get some medical attention, doctors can mistake them for normal signs of aging.  Debilitating depression and illusions of grandeur are not, however, a normal part of growing older. Treatment can be just as helpful for older adults as it is for every one else.  However, older adults may be taking other medications as well, and they should be made aware of possible interactions and side effects.  

Thanks to recent advances in technology, genetic research on bipolar disorder has become quicker, easier and more ambitious than in the past.  In August 2007, the National Institutes of Mental Health participated in the launch of the Bipolar Disorder Phenome Database.  Scientists are using this database to discover relationships between visible signs of the disorder and the genes that may influence those symptoms.   Among their most recent findings are that most people with a bipolar diagnosis have been treated or hospitalized for bipolar disorder, missed work because of their illness, and experienced other illnesses at the same time, particularly substance abuse and panic disorders.  

Among the traits that seem to run in families are histories of psychiatric hospitalization, and a tendency towards OCD (obsessive-compulsive disorder, another mental illness) Family members who shared a diagnosis of bipolar disorder had other, specific similarities.  They had their first manic episodes at around the same age, and they had a similar number and frequency of manic episodes. 

The increasing power and sophistication of MRI (magnetic resonance imagery), as well as new tools like functional magnetic resonance imaging (fMRI) and positron emission tomography (PET), have made it possible for scientists to conduct brain-imaging studies.  By taking timed pictures of the inside of the brain, scientists can understand what happens in the brain of a person with bipolar disorder.  Research so far suggests that bipolar brains differ from the brains of healthy people or people with other mental disorders.   

For example, a 2007 study reported on the website Bipolar-Lives revealed that the brains of people with bipolar disorder shrank more quickly than healthy brains. "Imaging studies carried out four years apart," the site reports,  "showed loss of brain tissue in the areas controlling memory, face recognition and co-ordination."  Bipolar patients who took lithium, one of the standard medications for bipolar disorder, reversed this decline in brain tissue.