As babies mature, they develop new perceptual, cognitive and motor abilities. These new abilities change how the baby relates to the world, often in a very fundamental way. Developmental scientists and doctors often refer to a baby as progressing through developmental stages to describe these changes. The most commonly used terminology for describing stages of baby development comes from Jean Piaget, a Swiss psychologist who lived between 1896 and 1980.
Reflex Schemas (0-1 month)
When Piaget was a new father, he spent a great deal of time playing with his son and noting the things that he did. His developmental theories grew out of these observations. For instance, Piaget found that the earliest patterns of infant behavior could mostly be described in terms of inborn reflexes. If you press your finger into a newborn's palm, she will squeeze it. If you brush your hand along one of her cheeks, she will turn in that direction. If a nipple is placed into her mouth, a sucking reflex will be engaged, enabling her to eat.
Primary Circular Reactions (1 to 4 months)
Piaget noticed that beyond 1 month of age, babies seem to repeat their reflexive behaviors in an intentional fashion. For instance, a 1-month-old baby might initially place his thumb into his mouth and suck on it by accident. Over the subsequent minutes, however, he may repeat this behavior several times in a row, presumably because he liked the outcome. This seemingly intentional repetition is referred to as a primary circular reaction.
Secondary Circular Reactions (4 to 8 months)
By 4 months of age, babies perform frequent primary circular reactions. As they enter this next stage of development, babies begin to direct those behaviors at external objects, engaging in what Piaget called "secondary circular reactions." One of the clearest behavioral milestones that emerges in this stage is a newfound ability to grasp a nearby objects, pick them up, and put them into the mouth.
Coordinated Secondary Circular Reactions (8 to 12 months)
Between 8 and 12 months of age, Piaget noticed that infants combine their secondary circular reactions in novel ways, in order to achieve certain goals. For instance, babies like to hear rattles make noise. Once an infant reaches this stage, she will be able to recognize an object as a rattle--even if she has never encountered that particular rattle before. She will grasp the rattle and shake it in order to hear the noise. Around this age, babies also typically start to crawl. As they do, they become independent explorers of their environment. Again, the relation between the baby and her environment changes in a fundamental way.
Tertiary Circular Reactions (12 to 18 months)
Once a baby reaches 12 months of age, several salient behavioral changes are apparent. At this age, babies typically stand up and walk. They also begin to use spoken words as symbols for familiar objects. Technically, once the child starts to walk, she sheds the title of baby and become a toddler. Piaget described children at this age as being like little scientists. They engage in trial-and-error explorations of the world around them. For instance, they may experiment with many different behaviors and sounds to determine what is the best way to get their parents' attention.
