Although they have long noted that depression and bipolar disorder run in families, experts continue to debate how much of that can be credited to nature and how much to nurture. Studies of twins and adopted children, plus a wealth of research from the Human Genome Project and the Human Genetics Initiative at the National Institute of Mental Health, have begun to answer some of these questions.

The most obvious genetic connection lies in bipolar disorder. Most experts believe it affects 1 percent of the general population, although some preliminary evidence suggests it could be even more common. But if a first-degree relative -- that is, a parent, sibling, or child -- experienced full-blown mania, you would have a 12 percent chance of developing bipolar disorder during your lifetime. If both of your parents had this illness, your chance of developing it would be 75 percent. Studies of identical twins, who share a genetic blueprint, show that if one twin has bipolar disorder the other has a 60 percent -80 percent chance of developing it, too. These numbers don't apply to fraternal twins, since, like other biological siblings, fraternal twins share only half of their genes.

The genetic components of depression are far harder to pin down. A person who has a first-degree relative who suffered a major depression would have a 1.5 percent to 3 percent higher than normal risk of experiencing one as well. But researchers have found it quite hard to sort out the actual influence of genes versus upbringing or other factors.

Thus far, experts say no single gene or combination of genes acts alone in triggering bipolar disorder or any form of depression. Rather, these illnesses probably result when genetic variations that create vulnerability interact with and are amplified by environmental factors, including early losses or chronic stress.

From the Harvard Health Publications Special Health Report, Depression Report. Copyright 2002 by President and Fellows of Harvard College. All rights reserved.