Twin Lateralisation: Biology and Psychology

The papers included in this volume extend the literature on lateralisation in twins, and reinforce the need to consider the lateralisation of twins from biological, neurological and psychological perspectives.

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Twins present special problems for the study of lateralisation, both psychological and biological. Central to both areas is the common observation that identical (monozygotic) twins can differ in their laterality. In the case of hand preference, this is most obvious in the one in five twin pairs where one is right-handed and the other left-handed. A similar problem can be found morphologically, not only in normal anatomical traits, such as hair whorls, which can be in opposite direction, but also in the rare condition of conjoined twins, where it is frequently the case that one twin has its heart and viscera in the normal orientation (situs solitus) and the other is mirror reversed (situs inversus).
This special issue of Laterality brings together papers on these different aspects of twinning. Two of the papers are biological, one providing a detailed analysis of the laterality of artificially conjoined twins created by fusing two different species of newt, and the other overviewing the recent developments in understanding the molecular biology of visceral lateralisation in relation to twinning. The other papers concern aspects of cerebral lateralisation in twins, one providing a much needed meta-analysis and critical overview of the twin literature, another providing detailed data from a large study of handedness in twins, a third asking about the almost neglected question of footedness, earedness and eyedness in twins, and the fourth looking at lateralised cognitive processing. Between them these papers extend the literature on lateralisation in twins, and reinforce the need to consider the lateralisation of twins from biological, neurological and psychological perspectives.