One of the most consistent findings in people suffering from MDI is a history of MDI, mood disorders and/or substance abuse in blood relatives.
A first-degree relative (parent, sibling or child) of a patient with Mania has a risk of suffering from Mania 8-18 times greater than a normal control (someone without a family history of Mania).
If one parent has Mania, the risk for the offspring is 25%. This means that 1/4 of children are likely to be ill. If both parents have a history of Mania, the risk for the offspring raises to 50-75% (2-3/4 of children are likely to be ill).
A study by Cytryn of 30 children (6-15 years old) of 14 families where one parent had a history of Mania or recurrent Depression found that 57% suffered from Depression, a common initial presentation of MDI (onset). This is also seen among children of patients with Mania that have been raised in adoptive families.
Twin, adoption and family studies have provided evidence that vulnerability to this illness is transmitted genetically.
Twin studies compare the rate of MDI in identical twins, which share the same genes, with that of fraternal twins, who are siblings of the same age. The concordance rate is the presence of MDI in the twin of a patient with MDI or the absence of illness in the twin of a healthy person. For identical twins (monozygotic, MZ) the concordance rate is 67% (33-90%), while for fraternal twins (dizygotic, DZ) is 20% (5-25%).