There's no doubt that there is a genetic component to the risk of developing mental illnesses. Several studies have confirmed this. It's been found that people with unipolar depression, bipolar disorder and schizophrenia are highly likely to have siblings, half-siblings and children to have one of these disorders or even have symptoms of more than one.
Studies of identical twins show that if one twin has bipolar, the other has very high risk of developing bipolar as well. There is also an increased risk within fraternal twins, but it is much lower than for identical twins.
What about more remote relationships? There are no studies, but here's a look at a family where four cousins are all diagnosed with bipolar disorder (names changed to protect privacy).
One Family's True Story
My friend Janet, who has bipolar disorder, also has three cousins on her mother's side with bipolar. Linda and Stacey are the children of an uncle and Jeff the child of an aunt.
Janet doesn't know everything about the family history, but was curious enough to put together what she does know in the chart below.
As you can see, the only known factor the four cousins have in common is Janet's maternal great-grandfather, who was diagnosed with "melancholia" (an old term for depression) and committed suicide.
Janet herself, the only one who had childhood bipolar symptoms, also has a history of mental illness on her father's side.
Is this a stretch? I don't believe so. Even though all four cousins also have siblings with no known signs of mental illness, it seems more than a coincidence that the four all have bipolar disorder.
So what about you? Does mental illness run in your family? Tell us about in in a comment.
McGuffin, P, et al. The heritability of bipolar affective disorder and the genetic relationship to unipolar depression. Archives of General Psychiatry. 2003 May;60(5): 497-502.
Lichenstein, P, et al. Common genetic determinants of schizophrenia and bipolar disorder in Swedish families: a population-based study. The Lancet. 17 Jan 2009. 373(9659) 234-239.
Edvardsen, J, et al. Heritability of bipolar spectrum disorders. Unity or heterogeneity? Journal of Affective Disorders. March 2008. 106(3) 229-240.