Imagine that you visit your older sister in a neighboring state just after she has had her first child. You see the infant as a newborn, just 1 week old. He does not really respond to you unless you place a bottle in his mouth or your finger in his hand. His movements include seemingly random flapping of the arms and legs—unless he is hungry, in which case he flails his limbs and cries! When you leave, you think, “He seems uncoordinated and weak.” After 9 months, you visit your sister and nephew again. What a change! He sits up on his own, reaches for toys, and has started to crawl. He can even stand when you hold him. He begins to coordinate his actions so that he can move purposefully. Now, let’s say you visit again, another 9 months later. Your nephew is no longer an infant but a full-fledged toddler. He can now walk—pretty quickly when he wants to—and has no problem with reaching and grasping. He is beginning to respond to language, particularly with the word no. He seems so very different from the newborn you met a mere 18 months before!
In this scenario, it would be natural to wonder, “What happened during the past year and a half that resulted in these changes?” In other words, how can you (or anyone else) explain the changes seen across development (figure 2.1)? Given that there appear to be similarities in the development of different people (universality, described in chapter 1), how do we organize and understand these changes so that we can explain them and predict future development? Certain facts exist. How can we make sense of them? We must look at the different theoretical perspectives on motor development; theories provide a systematic way to look at and explain developmental change.