This is a very interesting analysis and the largest and most definitive study to date. The results make clear that

1. some genetic effects are shared between BPD and schizophrenia;
2. some genetic effects are unique;
3. some environmental effects are shared;
4. some environmental effects are unique.

What is not known is which G or E effects validate diagnostic class. And whether all the effects are small, and apply to subgroups, and, therefore, may be more useful in resolving syndrome status than suggesting BPD and schizophrenia are one disease. The one-disease-or-two debate only has meaning if there are two diseases at most. More likely, we have two syndromes with some overlapping subjects, some overlapping psychopathology, some overlapping phenotypes.

The key question is whether to have the two syndromes as the unit of analysis for most studies. This can lead to new information on unique and shared effects, but the heterogeneity of the syndromes will confound this approach.

An alternative approach is the "domains of pathology" approach, which...  Read more

View all comments by William Carpenter

The results of the family/adoption study by Lichtenstein et al. (2009) and our twin study (Cardno et al., 2002) are remarkably similar. Using a non-hierarchical diagnostic approach, the genetic correlation between schizophrenia and bipolar/mania was 0.60 in the family/twin study and 0.68 in the twin study. The heritability estimates were somewhat lower in the family/adoption (~60 percent) than twin study (~80 percent), but can still be said to be substantial and similar for both disorders.

When we adopted a hierarchical approach, with schizophrenia above mania, we found no monozygotic twin pairs where one twin had schizophrenia and the other had bipolar/mania, but with their considerably larger sample, Lichtenstein et al. (2009) were able to confirm a significantly elevated risk for bipolar disorder in siblings of probands with schizophrenia (RR = 2.7), even when individuals with co-occurrence of both disorders were excluded.

I think there is a potentially interesting link...  Read more

View all comments by Alastair Cardno