Development of role-differentiated bimanual manipulation in infancy: Part 1. The emergence of the skill.

This is the first paper in a series of three discussing different aspects of the development of role-differentiated bimanual manipulation (RDBM-two hands performing different but complementary actions on an object). Emergence of RDBM is an important shift in the development of infant manual skills. Trajectories of monthly changes in the number of RDBMs and RDBM hand preference were explored in a sample of 90 (57 males) normally developing infants (30 with a right preference for acquiring objects, 30 with a left preference, 30 with no preference) during the 9-14 month period. Multilevel analysis revealed that infants performed more RDBMs with age, with similar patterns of change in all hand preference groups. A hand-use preference for RDBM became more prominent with age with most right-preferring infants and those without a preference for object acquisition developing right-hand preference for RDBM. Left-preferring infants exhibited more heterogeneity in their hand-use for RDBM.