 The study involves one 30-minute session with the baby and his or her mother or father. The parent will answer some questions about the family, such as its size and ethnic make-up. The infant will then have small magnetic sensors taped to the underside of each arm and to the back. While sitting on the parent s lap, the infant will be presented with plastic toys, and his or her actions will be measured by the sensors as he or she reaches for and picks up the toys. The sensors will be connected to a computer that will track and record the motion of the infant s arms. 