BACKGROUND Bulimia nervosa is typically defined as the combination of the behaviours of binging and vomiting. We sought to clarify the relationship of these behaviours from a genetic epidemiological perspective.

METHOD Using data on the lifetime history of binging and vomiting from a personally interviewed population-based sample of female twins (n = 1897), we applied bivariate twin modelling to estimate the sources of variation for these traits.

RESULTS The association between having ever binged (23.6%) and having ever induced vomiting (4.8%) was very strong (odds ratio = 8.78, P < 0.0001). The best-fitting model indicated that lifetime binging and vomiting were both heritable (46% and 72%) and influenced by individual-specific environmental factors (54% and 28%). The overlap between the genetic (ra = 0.74) and individual-specific environmental factors (re = 0.48) for the two traits was substantial. No violations of the equal environmental assumption were evident.

CONCLUSIONS Including binging and vomiting under the rubric of bulimia nervosa appears to be appropriate. Our data are consistent with the identification of binging and vomiting as complex traits resulting from the interplay of multiple genes and individual-specific environmental influences. In contrast to 'environmentalist' theories, our results suggest that genetic influences may be of particular relevance to the aetiology of binging and vomiting.