Evaluation of nature-nurture impact on reproductive health using half-siblings.

Research in the field of preventive medicine will increasingly focus on the role of genetic susceptibility in disease etiology. Epidemiology plays an important role in identifying which diseases are good candidates for such research activity. Computerized population registries of unstable partner relationships and change in environmental exposure settings may provide new tools for research. We illustrate these tools using facial cleft defects as an example. The design is based upon computerized and stored data from large population samples. Data on change of partner or environment between births are used to learn about the recurrence risks for diseases that were present in their first child. The study focused on a susceptible subgroup of the population who previously had an affected child. Thus, by definition, these couples had a sufficient set of causes to initiate the disease and an increased risk of recurrence if relevant genetic or environmental factors remained unchanged. When considering recurrence risks after changes in possible genetic or nongenetic risk factors, etiologically important clues may emerge. The example confirms that genetic factors play a major role in facial cleft defects.