Abstract
BACKGROUND: Although the genetic and environmental factors of asthma have been investigated in adolescence and adulthood, no previous studies have focused on the early development of asthma. AIMS: To test, in a large sample of 4 year old twins, the hypotheses derived from the literature on adolescents and adults that genetic influences are substantial and shared environmental influences are modest. METHODS: The sample consisted of 4910 twin pairs who were born in England and Wales in 1994 and 1995. Data on asthma status were obtained from the twins' parents by postal questionnaire. RESULTS: Univariate parameter estimates derived from model fitting were 68% heritability, 13% shared environment, and 19% non-shared environment. CONCLUSIONS: Our findings suggest that asthma is highly heritable in 4 year olds, whereas shared environmental influences are not statistically significant.