Milestones Of Development

Human development is a complicated affair, progressing as the result of the continuous interaction of biologic and environmental factors. It is for this reason that no two people are exactly alike, not even identical twins. Despite such variability, there are aspects of development that are predictable, such that children throughout the world develop certain abilities and characteristics at about the same time. These universal accomplishments are termed "milestones"—guideposts that reflect normal, species-typical development.

The temporal regularity of these milestones implies that they are under biological control, little affected by the vagaries of the external world. This is only partially true, for all aspects of development are also influenced by environmental factors. Children inherit not only a species-typical genome (DNA), but also a species-typical environment, which begins prenatally and continues after birth as infants around the world are nurtured by adults in social settings. Subtle differences at both the genetic and environmental levels affect development of even these reliable milestones, so that experts are not able to specify the exact time children will display a particular characteristic but can state only approximately when they will appear. Variation around these average times is normal, with half of all children showing these characteristics sooner than average and half later than average.

In Tables 1-3 are partial lists of physical, cognitive, and social/emotional milestones, denoted separately for the periods of infancy, preschool, school age, and adolescence. Some of these milestones have great social significance. For instance, in some traditional societies, a girl's first menstrual period signals a move from childhood to adulthood; and, in American society, being out of diapers is a requirement for admission to some preschools.

Perhaps the first thing to note is that there are many more entries for infancy than for the other age groups. This is primarily because the accomplishments of the first two years of life are more under the influence of maturational factors than environmental ones. As children get older, their developmental pathways vary as a function of the societies they live in. For example, for children in literate societies, one could have included milestones related to reading. Reading, however, requires specific instruction that not all children receive; moreover, there are different writing systems, alphabets, and educational philosophies that result in different patterns of reading-related behavior even in literate cultures.