Use of twins to study environmental effects.

Extrapolation from pharmacogenetic studies would indicate that there is a great deal of genetic variability in the response of humans to noxious environmental agents. Co-twins provide the most closely matched genetic controls possible and, in addition, are matched perfectly for age and often have shared very similar environments since before birth. The efficiency of co-twin control studies can be further increased by the use of sequential analysis, so that for the studies of the effects of environmental agents on human populations, twins would give answers that would require many more unrelated subjects. Use of twins in epidemiology studies is now in its infancy, but investigators should carefully consider the use of this powerful experimental tool and begin to identify twins in population surveys.