Bipolar disorder (or manic-depressive disorder) is a severe mood disorder in which episodes of (hypo) mania (e.g. elevated mood en hyperactivity) and depression (e.g. decreased mood and reduced activity) alternate with periods of normal mood and functioning. Genetic factors as well as environmental factors play a role in the development of bipolar disorder. A serious problem for bipolar disorder is a delay of several years between the first mood episode(s) and the diagnosis. Afterwards it can take several years before starting the right treatment. This twin study shows that the genetic vulnerability for bipolar disorder can appear in several ways and in several phases of life. •Early during intrauterine life between the 10th and the 17th we...