In order to separate genetic and environmental contributions to liability for schizophrenia and bipolar disorder, we analyzed outcomes from different family types: nuclear, paternal half-sibling and maternal half-sibling families. To reduce complexity in families with half-siblings, the marriages of the index parent were limited to two spouses. This excluded 8% of the half-sibling families. When more than two children were available, we analyzed only the oldest two full siblings from a nuclear family and the oldest two half-sibling from a half-sibling family. Our data include 1,984,182 nuclear families, 172,073 paternal half-sibling families, and 161,409 maternal half-sibling families.
