The Tonic Neck infant
reflex is an automatic response that
babies have up until about four months of age. If you turn a child's head to the side while he is lying
awake on his back, he will reach one arm
forward in the direction his eyes are pointing, and the other arm will
flex.
Scientists don't really know why all newborns behave this way, but it may be a step in the development of the reaching and grasping motion.