The validity of twin studies


: Prof. N. G. Martin, Queensland Institute of Medical Research, PO Royal Brisbane Hospital, Brisbane 4029, Australia


The classical twin study is the most popular method for assessing the relative contribution of genes and environment to traits in human populations. Critics argue that several assumptions of the twin method are unjustified, and therefore results from twin studies are misleading. Specifically, it has been suggested that twins differ in important aspects from singletons, that monozygotic (MZ) and dizygotic (DZ) twins are not matched in their degree of environmental similarity, and that MZ twins are neither matched genetically nor in their prenatal environments. These criticisms are addressed and it is suggested that they do not provide serious impediments to the validity of the twin study.