We further considered a multivariate logistic regression model, conditional on pair membership, to estimate odds ratios (ORs), with 95% CIs, as measures of relative risk of a fracture in the second twin if the first twin had had a fracture. The etiologic fraction was calculated by the following formula: ([OR − 1]/OR) × p, where p is the number of twins with a co-twin who had a fracture divided by all twins with fractures, which equals the probandwise concordance. This is an estimate of the proportion of all fracture cases that could be attributed to a twin partner with a fracture. Larger differences in etiologic fractions between monozygotic and dizygotic twins indicate a stronger genetic component. In the multivariate model we included, in addition to age at the end of the follow-up period, body mass index (calculated as weight in kilograms divided by the square of height in meters) and menopausal age (all continuous), hormone therapy (ever vs never), smoking status (never, former, or current), and leisure-time physical activity (low, medium, or high) from the telephone interview. From the central registries, we further included dichotomous variables reflecting malignancies, endocrine disorders, cardiovascular diseases, psychiatric disorders, neurologic disorders, gastrointestinal disorders, musculoskeletal diseases including rheumatoid arthritis, and renal failure or other urinary tract diseases. The primary purpose of the multivariate adjustment was to evaluate whether differential changes of the ORs for the zygosity groups could be detected. Differential changes in the ORs after multivariate adjustment and, hence, attenuated differences in etiologic fractions between monozygotic and dizygotic twins may suggest that the age-adjusted heritability estimates are inflated by the risk factors included in the models.