Use of the correlation of liability in twins and siblings in the study of birth defects.

The epidemiologic approach to determining the etiology of disease involves identification of potential risk factors and then comparison of disease incidence among people with varying levels of exposure to the potential risk factors. This paper defines risk factors which correspond to different levels of genetic and environmental proximity to index cases of birth defects. Genetic proximity is estimated by the coefficient of relationship (R): 0.5 for siblings and dizygotic twins and 1.0 for monozygotic twins. Environmental proximity is measured by a combination of two variables: one variable for those potentially preventable risk factors common to siblings (S) and another for those common only to twins (T). Discordance in identical twins is attributed to a third type of environmental factors (U) that are unshared by twins and include random (stochastic) factors. The association between these risk factors and birth defects is estimated by using a linear model of the correlation of liability for different relatives. The coefficients derived from the model reflect the relative importance of genetic and different types of environmental risk factors as causes for the defects and can be used to identify birth defects most likely to be caused by measurable and possibly preventable risk factors. These defects could then be assigned high priority for future studies and preventive efforts.