Service implications of the Calman report.

resulted in a more proximal location for the putative gene. The trade off, however, was a prevalence in the general population similar to the familial rates in the sample overall. In other words, the evidence for familial aggregation, the cornerstone of a genetic hypothesis, was attenuated. There are lessons too from other studies-for example, of the hypothesis that another behavioural trait, manic depressive illness, is X linked. Support for this hypothesis was initially furnished by segregation patterns consistent with X linked transmission and reports of linkage to chromosomal region Xq27-28. In some studies the statistical support for these findings far exceeded the significance levels reported by Hamer et al. ' Moreover, the evidence from twin and adoption studies for a genetic component in manic depressive illness was far more compelling than that for homosexuality. Unfortunately, non-replication of the linkage findings by other investigators, as well as extension and reevaluation of the original data, has resulted in diminished support for this hypothesis.8 This outcome underscores the uncertainties in linkage studies of complex behavioural traits. The claim of linkage ofmale homosexuality to chromosome Xq28 has wide social and political implications. Yet the scientific question is a complex one, and the interpretation of these results is hampered by methodological uncertainties. Further study is crucial to confirm or refute this finding.