Although doctors and research scientists seem obsessed with mapping the human genome and identifying possible genetic links to every affliction from Alzheimer’s to zits, it’s parents and children who tend to be most concerned about bipolar disorder and heredity. Parents often ask me how likely it is that their children will develop bipolar disorder if someone in the family has it. And one of the biggest sources of anxiety for children of parents with bipolar is whether they will eventually “get it.”

Although some studies have produced statistics to help answer this question, the fact is that for any given individual, there’s no accurate way to predict the eventual onset of bipolar disorder, because it’s not determined solely by genetics. Someone may have a genetic vulnerability to developing bipolar, but that doesn’t doom the person to eventually developing it.

Bipolar Genetics – The Statistics

For those who want the statistics, here they are. Take these with a huge block of salt, however. Results differ depending on the study, and as I’ve said, any statistics cannot predict with any certainty what will eventually happen concerning a particular individual.

• One percent of the general population has Bipolar I.
• Two to three percent of the general population likely has Bipolar II, and an even larger percentage is thought to have Cyclothymic disorder.
• If one parent has bipolar disorder a child has about a 12-15% chance of developing the disorder. If both parents are diagnosed the risk climbs to about 25%.
  • One study, done in 1997, put together data from 17 previous studies, and they determined that if a child had a parent with bipolar disorder they were 2.7 times more likely to develop bipolar disorder than if the parent did not have bipolar disorder. Children of people with bipolar disorder were 4 times more likely to develop some type of mood disorder, not necessarily bipolar disorder.
  • In situations in which the person diagnosed is a generation or two (or more) removed, the numbers are fuzzy. The most important risks are in immediate, first degree relatives. There are studies that have identified some specific cognitive, emotional, and behavioral symptoms that occur more frequently in relatives of people with bipolar disorder even when these relatives do not have a full diagnosis of bipolar. These studies are increasingly important in the search for the genes that do increase the likelihood of developing bipolar disorder.
• In about 80% of cases involving identical twins, if one of the twins is diagnosed with bipolar, the other is, too, which provides pretty clear evidence that the illness is strongly heritable – highly related to genetic loading – but it also tells us that the onset is not completely driven by genetics.
• With fraternal twins, in about 25% of cases when one twin is diagnosed with bipolar disorder does the other twin eventually develops it. The risk is closer to 12 to 15% in non-twin siblings.

Is There a Single Bipolar Gene?

The short answer to this question is no. Current research indicates that there are at least three to four genes, quite possibly more, involved in the development of bipolar disorder.

• Multiple genes appear to be involved in creating a vulnerability to bipolar disorder, and they overlap with genes suspected of causing other psychiatric disorders, including unipolar depression and schizophrenia. See “Similar Genes Increase Risk for Bipolar and Schizophrenia,” by Psych Central Senior News Editor Rick Nauert.
• Multiple disorders (from different problems in different parts of the brain) are likely to contribute to the symptoms doctors collectively refer to as bipolar disorder. This means the genetic search is actually exploring several disorders rather than just one. The people under the microscope may have a range of bipolar subtypes, making the results confusing and inconsistent.
• Identifying and locating the genes is complex and difficult work. Some of the most interesting research involves exploring rare single gene mutation syndromes that present with bipolar symptoms to help further understand which genes may be involved. Studies are also looking at certain families that have particularly strong occurrences and associations with bipolar disorder, because this can give rich information about the genes associated with bipolar disorder.
• Clearly genes are not enough to cause bipolar disorder in most cases. Environment is clearly involved and research is looking into many possible culprits – and this is part of the genetic research as well.

Can I Improve My Odds?

Studies consistently show that genetics makes up about 75-80% of the cause of Bipolar I disorder. (This means that if someone has bipolar disorder, the cause of it is 75-80% due to genetics. It does not mean an individual has a 75-80% chance of developing it.) Environmental variation may play a larger role in other bipolar subtypes, but the jury is still out on this.

With genetics contributing up to 80% to the development of bipolar disorder, this means that 20% or more is due to environmental agents and events. It’s unclear when these factors need to occur to trigger the onset of bipolar disorder – some of the major environmental contributors probably occur in early brain development including prenatal and early life.

What sorts of environmental agents and events can possibly trigger the onset of bipolar disorder? Is there anything that can be done to prevent bipolar in someone who has a potential genetic vulnerability to it? These are the questions we address in Part II of this series. Tune in this coming Tuesday for the answers.

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    Last reviewed: 25 Jul 2008