The fact that twins are more likely to share environmental factors can be overcome if twins or siblings that have been adopted are studied as they will be exposed to a different environment to their biological or genetic relatives.
Adoption studies involve looking at a disease in biological or genetically related relatives in comparison to adoptive relatives who are genetically different but who share the same environment as the adoptee. If genetically related relatives show higher degrees of concordance for a disease than adoptive relatives, this suggests that genetic factors are important in causation. If concordance is greater in adoptive relatives, this suggests that environmental factors are more important in disease causation.
To view this interactive tutorial, you need the latest Flash player.