Twin-Singleton Differences in Intelligence?

Abstract

Introduction Classic behavioural genetic studies provide statistical estimates of heritabilities that form the first step in the search for genes for complex behaviour. 1,2 A large part of these behavioural genetic studies are based on twin samples. These samples have sometimes been criticised for their alleged non-generalisability; since twins are `special' they may not be representative of singletons. Especially in the field of cognitive abilities twins are generally considered to be at a disadvantage compared with singletons. 3--6 Twins share the womb at the same time and consequently share prenatal nutrition provided by the mother's dietary intake. When preparing for labour, twins compete for the best position. This suboptimal intrauterine environment may lead to prematurity, low birth weight and lower weight-forgestational age, 7 which in turn in several cases have been associated with low childhood IQ. 8--12 Apart from a general suboptimal intrauterine environment for b