Handedness for bimanual coordinated actions in infants as a function of grip morphology.

We investigated the emergence of bimanual handedness in tasks involving complementary roles for the two hands, one hand holding a base object and the other hand removing several pieces from the base object. Infants aged 12, 16, and 20 months were tested on bimanual tasks differing mainly in the precision of the movement required to remove the pieces. The results show that the right hand was more often used than the left hand not only to grasp the base object but also to remove the pieces, often after transferring the base object from the right to the left hand. As of 12 months of age, right hand preference for the active part of the bimanual task was stronger in the precision grip than in the whole-hand grip tasks. These results indicate that even though infants often do not anticipate that they will need their preferred hand to remove the pieces, they show clear handedness in such coordinated repeated bimanual actions, and do so to a greater degree on tasks requiring precision grip than on ones requiring whole-hand grip. These results agree with the notion that handedness develops very early and is related to the precision required from the active hand.