NO EVIDENCE FOR EFFECTS OF FAMILY ENVIRONMENT ON ASTHMA - A RETROSPECTIVE STUDY OF NORWEGIAN TWINS

The risk of developing asthma contingent upon the co-twins' history of asthma was analyzed in a population-based study of 5,864 Norwegian tw ins. A primary aim was to assess the significance of shared environmen t for the development of asthma from infancy through age 25. Retrospec tive reports were collected when the twins were 18 to 25 yr of age. Th e risk of developing asthma, contingent upon the co-twin's history of asthma, was estimated using survival analyses, and genetic and environ mental sources of variation in liability for asthma were analyzed with structural equation models. The cumulative incidence of asthma was 6% for males and 5.4 % for females. The relative risk of developing asth ma among twins whose co-twin had a positive history of asthma compared with those whose co-twin had no history of asthma was 17.9 (95% CI, 1 0.3 to 31.0) for identical, and 2.3 (95% CI, 1.2 to 4.4) for fraternal twins, Although shared environment encompasses many of the exposures that are putative risk factors for asthma in this age range, there is no evidence of shared environmental influences for asthma. Rather, 75% of the variation in liability for asthma was explained by genetic eff ects and the remaining variation was due to nonshared environmental in fluences. These results suggest that the familial risk for asthma is p rimarily, genetic.