Evidence for the Aetiological theories: Genetic
When identical twin pairs are screened for an illness, and this is compared to rates of that same illness in non-identical twins, its possible to see how strongly genes contribute to susceptibility to that illness.
The comparison of the “genetic loading” of Bipolar vs. Unipolar Depressive disorders suggests that Bipolar Disorder is more heritable. However, since concordance doesn’t even come close to 100% in Bipolar Disorder, it also shows that other non-genetic factors must be involved.