Differences in protocols of craniofacial development related to twinship and zygosity.

Using 56 adult dental diameters as a subsystem model for craniofacial development, we show that monozygotic (MZ), dizygotic (DZ), and singleton groups differ significantly in developmental relationships assessed by multivariate statistical methods under commonly accepted assumptions. Given the differences observed, we suggest that any assumption of developmental equivalence between MZ and DZ twins, or between twins of either group and singletons, for variables of craniofacial or behavioral development, may be subject to serious doubt. Implications for twin study theory and methodology, and for study of early human development, are discussed.