Lateralised behaviour in first trimester human foetuses.

Behavioural lateralisation is a common feature of everyday behaviour, most familiar in the exhibition of handedness. Despite differing theories about the origins of laterality, little is known about their ontogenesis. This study observed the left and right arm movements of 72 foetuses at 10 weeks of gestational age, the first age at which lateralised behaviour may be observed. There was a highly significant preference for foetuses to move their right arm more than their left arm, 85% exhibiting more right arm than left arm movements. There was no difference in the amount of movements exhibited with the preferred arm, whether it be right or left. Laterality of behaviour at such an early gestational age (probably as early as it is possible for it to be observed) is unlikely to be under brain control but is probably of more muscular or spinal determination. The possibility that early differential motor behaviour may contribute to subsequent laterality of behaviour and asymmetrical brain growth, must be considered.