Origins and implications of handedness and relative birth weight for IQ in monozygotic twin pairs

Segal NL

1989

USA

The results of this study support the hypothesis that left handedness in lower birth weight identical twins may be associated with pre-natal pathological events, while left-handedness in higher birth weight left handed identical twins may be associated with delayed zygotic splitting and disrupted asymmetry determination. Birth weight status within identical twins may predict a lower IQ for the lower birth weight twin only when lower birth weight co-occurs with left handedness. Prenatal insult similarly reduced verbal IQ, but not performance IQ, within these particular pairs.

The authors did not compare first born twins and second born twins. The birth of a second twin is often more difficult. This would be a way to evaluate the importance of perinatal factors.