If we want to understand their behavior, we have to learn about the sequence of these changes, and what to expect at different stages of development. I use a developmental approach to understand a child's current functioning, and to understand how a certain strength or weakness has developed over time. Some of the most visible milestones are sitting up, crawling, walking, and talking.
Many skills have to be in place before a child can accomplish the most visible milestones.
For example, the child must be able to control the head and trunk in order to sit independently. A child who is not walking by the usual age may not have a significant developmental problem if the skills leading up to walking are well-developed. However, a child who is not walking and hasn't established head control is facing much more serious difficulties. Behavior and feelings have different meanings depending on the child's age and developmental profile. The stages of emotional, social, and personality development are generally less well understood than milestones of motor or language development.