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.

Physical Milestones

The list for physical development (Table 1) includes a number of familiar milestones for infants, most related to gaining control over their bodies so that they are able to move about on their own. The list of milestones for the preschool years will also be familiar. It is during this time that children become toilet trained and learn to use simple tools, such as forks and spoons. The first permanent teeth erupt around six years of age. Although children across the globe are typically weaned by age three or four, they are not able to eat an adult-style diet until they have most of their permanent teeth. This means that adults must specially prepare food for children years after they have stopped nursing. This is a pattern seen in no other animal and makes the period of "childhood" unique to the human species.

Physical growth is slow and gradual between the ages of about six and eleven, when the adolescent growth spurt begins (sometimes a bit earlier for girls). The rapid growth at this time, which occurs later for boys, coupled with the development of secondary sexual characteristics, marks the physical transition to adulthood. Girls' first menstrual period (menarche) usually occurs about two years after the onset of secondary sexual characteristics, and both boys and girls have a period of relative infertility, lasting several years, after they have become sexually mature. Although the pattern of puberty described here is universal, the average age at which girls reach puberty has been decreasing over the past two centuries, primarily because of better nutrition and health. There is also evidence that girls from high-stress, father-absent homes reach puberty earlier than girls from low-stress, father-present homes, reflecting the role that social factors can have on physical development.

Cognitive Milestones

Selected cognitive milestones are presented in Table 2. It is not until around seven or eight months that infants will search for an object hidden as they watch, believing, apparently, that the object continues to exist even though they no longer see it. First words are usually uttered late during the first year, and children's first two-word sentences are typically spoken between eighteen and twenty-four months of age. Language abilities develop rapidly during the third year of life, so that by age three and a half, most children are linguistic geniuses, being able to speak their native tongue proficiently (and far better than most adult second-language learners). Children have a difficult time taking the psychological perspective of others until about three and a half to four years of age. Until this time, they often believe that if they know something (for example, that a cookie has been moved from a box to a jar), other people should know it as well, even though others have different knowledge (not knowing the cookie was moved). Understanding that people's behavior is governed by beliefs and desires, which may be different from one's own, has been termed "theory of mind" and is the basis of all sophisticated human social interaction. Thinking becomes more logical during the school years, and this is perhaps best reflected by conservation tasks, developed by the Swiss psychologist Jean Piaget. Beginning around six years of age, children realize, for example, that the amount of water one has is the same regardless of whether the container that holds it is short and fat or tall and skinny. Much before this time, the appearance of "more" in the tall container determines children's thinking in such situations. With adolescence comes abstract thought, again as first described by Piaget. Children are able to think scientifically and are able to reflect upon what they already know.

Social/Emotional Milestones

Table 3 presents some social/emotional milestones. The social smile, observed early in infancy, reflects a general responsiveness to people, critical for an intensely social species such asHomo sapiens. Infants' attachment to their parents is sometimes reflected by a wariness of strangers and by distress when they are separated from their caregivers. By the pre-school years, children are able to identify emotions in others and can seemingly empathize with the feelings of others, as reflected, for instance, by a three-yearold bringing his tearful mother his teddy bear to comfort her. Although toddlers are interested in other children, friends typically do not become important until the early school years, at which time children enter the peer group and establish dominance hierarchies, often based on physical strength, especially among boys. Typically during this time, boys and girls segregate themselves into same-sex play groups. In adolescence, the peer group becomes increasingly important (although the family rarely loses its influence), and, coupled with the onset of puberty, heterosexual interests and behavior commence.

A milestone approach to development provides a quick glimpse at important acquisitions that children over the world experience. There is still much variability in when children attain these milestones, because of both biological (genetic) and environmental (societal) factors. And many culturally important phenomena that arise only with specific experiences (e.g., reading, religious practices) are not captured by knowledge of milestones. Nonetheless, milestones show what is universal in human development and give parents and educators an idea of how quickly children are progressing relative to a species-typical standard.

See also: STATES OF DEVELOPMENT; THEORY OF MIND

Bibliography

Belsky, Jay, Lawrence Steinberg, and Patricia Draper. "Childhood Experience, Interpersonal Development, and Reproductive Strategy: An Evolutionary Theory of Socialization." Child Development 62 (1991):647-670.

Bogin, Barry. Patterns of Human Growth, 2nd edition. Cambridge, Eng.: Cambridge University Press, 1999.

Wellman, Henry M. The Child's Theory of Mind. Cambridge, MA:MIT Press, 1990.

David F.Bjorklund

Jesse M.Bering