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...