Relationships between environmental and genetic risk factors in the aetiology of diseases and problems in the interpretation of case-control and cohort studies. Cholelithiasis as a model.

Gallstones are used as a model to individuate some theoretical grounds which can produce different outcomes between case-control or cohort epidemiologic studies and ecological studies. Incongruities take place when several independent causes are involved in the aetiology of a disease. These occur particularly when genetic and environmental causes are present and the latter bring out the disease in subjects bearing the genetic alteration. Under these circumstances (very frequent in the aetiology of chronic diseases) case-control or cohort studies are more likely to discover genetic risk factors of diseases since they compare subjects within a population that have more genetic than environmental differences. On the other hand, ecological studies tend to indicate environmental risk factors of diseases since they confront populations most often with greater environmental differences than genetic differences.