The description of a developmental stage considers what a baby can and cannot do and how she relates to the world around her. The most commonly used terminology for babies' developmental stages comes from the theories of Jean Piaget, a Swiss developmental psychologist who lived from 1896 to 1980. Piaget argued that children at different developmental stages conceive of the world in fundamentally different ways.
Reflex Schemas Stage (0 to 1 month)
Piaget developed his theory based on work conducted with his own three children. He carefully observed them as they interacted with him and the objects around them, and developed a theory that described their behaviors. At the earliest stage of development, Piaget claimed that babies behave based only on inborn reflexes. For instance, when you touch a baby's palm, he will close his hand and squeeze. If you brush your finger against a baby's cheek, he will turn his head in that direction. Piaget believed that babies succeed in eating because of a sucking reflex that is in place even before they are born.
Primary Circular Reactions Stage (1 to 4 months)
By 1 month of age, babies seem to use their reflexive newborn behaviors in an intentional fashion. For instance, a 1-month-old might initially put her thumb into her mouth and suck on it completely by accident. In the future, however, she will be much more likely to repeat the action sequence. In some cases, she may repeat it several times in a row, strongly suggesting that it is no longer accidental.
Secondary Circular Reactions Stage (4 to 8 months)
One of the most notable developmental advances of babies around 4 months of age is a new-found ability to grasp nearby objects, pick them up and put them into their mouths. Piaget described this as a secondary circular reaction. Like the primary circular reaction, babies repeat initially accidental sequences. In this case, however, the babies can direct those actions at external objects rather than just their own bodies. Children at this age also become adept at crawling--around 6 months of age. This independent control of movement is a large change in how babies relate to the world around them.
Coordinated Secondary Circular Reactions Stage (8 to 12 months)
Beyond 8 months of age, babies continue to exhibit intentional actions and begin to combine them in novel ways in order to achieve their goals. For instance, a baby will be able to recognize an object as a rattle--even if he has never seen that particular rattle before--and shake it in order to hear the noise.
Tertiary Circular Reactions (12 to 18 months)
Around 12 months of age, babies stand up and begin to walk. They also start to talk, using words as symbols to represent familiar objects. Once they start walking, they shed the title of infant and become toddlers. Piaget noted that toddlers use extensive trial-and-error at this stage to try to figure out how the world works--almost like little scientists studying a strange new planet. Many parents of children at this age realize they themselves are the subject of this experimentation, as their babies seek to determine just what types of sounds and actions are best for getting attention.
Other Systems of Stage Description
Much research has been conducted to flesh out the details of Piaget's theory of development, but other schemes have also been devised to characterize the babies' developmental stages. For instance, Sigmund Freud described developmental stages in terms of how humans experience pleasure at different ages. Freud believed that babies are within the oral stage of development. Others have referred simply to lists of ages associated with baby developmental milestones. For instance, babies enter the stage of object grasping at 4 months of age, crawling around 6 months, and walking around 12 months.
