“Offspring of Twin Pairs Discordant for Psychiatric Illness”, 1986-09-19 (; backlinks):
A refinement of the twin method is the investigation of the risk of similar affliction in the offspring of affected and non-affected MZ and DZ twin partners.
In the Danish twin studies on schizophrenia and on manic-depressive disorder, the morbid risks were of the same size in the children of affected and non-affected MZ twins, and of affected DZ twins, whereas the children of the normal DZ twins showed considerably lower risks.
Although the total numbers of affected offspring are small, the findings are in favour of a predominant genetic factor in schizophrenic and manic-depressive disorder.