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

The implications of left-handedness for intellectual functioning in monozygotic (MZ) twinships remain unresolved. Two proposed explanations of hand-discordance in MZ twin pairs, elevated birth stress and delayed zygotic splitting, were used to generate hypotheses and predictions concerning IQ, both within and between twin pairs. A comparison of Full Scale IQ, Verbal IQ and Performance IQ scores for MZ twin children, from 67 twin pairs, organized according to concordance or discordance for handedness and relative birth weight, is presented. The results support the hypothesis that left-handedness in lower birth weight MZ co-twins may be associated with pre-natal pathological events, while left-handedness in higher birth weight left-handed MZ co-twins may be associated with delayed zygotic splitting and disrupted asymmetry determination. Birth weight status within MZ twin pairs may predict a lower IQ for the lower birth weight twin only when lower birth weight co-occurs with left-handedness. Pre-natal insult similarly reduced Verbal IQ, but not Performance IQ, within these particular twinships.