ABSTRACT
Much research has been conducted on populations of young twins, of which most is oriented towards the genetic aspects of twinship. Monozygotic twins (MZ) are compared with dizygotic twins (DZ). Only a few empirical studies have examined the twin situation itself, comparing twins with singletons. During the past 30 years, several studies have shown a lag in the development of twins compared with singletons1-4, accompanied by a difference of opinion as to whether this lag is an inherent feature of twinning, or whether due to postnatal factors.
