INFANCY AND CHILDHOOD
Nine months after conception, the human fetus is ready to leave the uterus to enter the outer world. Ready, however, is a relative term. Most other animals can walk shortly after birth, and many can take care of themselves almost immediately. Humans, in contrast, are extraordinarily helpless at birth and remain utterly dependent on others for many years.
Why is human development so slow? Lions (as just one example) chase their male cubs away from the pride by age 2 or 3. Human parents, in contrast, care for their offspring for the better part of 2 decades (or more!). One might think that this would be a great disad-vantage for our species. It turns out, though, that this long period of dependency is ideal for a creature whose major specialization is its capacity for learning and whose basic invention is culture—the ways of coping with the world that each generation hands on to the next. Human infants, in other words, have a huge capacity for learning and a great deal to learn. Under these circumstances, there is much to be gained by two decades or so of living at home—even if this arrangement is at times inconvenient for child and parent alike.
In the following sections, we will consider three major aspects of development, including the infant’s sensorimotor development, her cognitive development, and her socioemotional development. We should be clear at the outset that the distinctions among these aspects of development are, to some extent, just a convenience for researchers (and textbook writers), because the various aspects of development plainly interact with each other. The child’s intellectual development, for example, is shaped by what she perceives and how she interacts with the world. Similarly, the child’s socioemotional development depends on her cognitive development, and vice-versa.
A similar point must be made regarding the interplay of genetic and environmental factors. These, too, constantly interact to codetermine a child’s developmental trajectory. For example, we mentioned that genetic factors are cru-cial for shaping a child’s intellectual functioning, but as we discussed, the environment also plays a huge role. Some environmental factors are biochemical (nutrients, toxins), and one tragic reminder of this comes from studies of mental retar-dation. For years, many types of paint contained lead-based pigments, and as the paint deteriorated, leaded dust would fall into the environment where it could be ingested or inhaled by children. Once inside the body, the lead interfered with the development of the nervous system, at large doses causing coma or even death, or at lower doses producing a litany of intellectual problems that often led to a diagnosis of mental retardation. This is why lead paint was banned for use in U.S. residences in 1978; other countries have similar (and older) bans in place.
Environmental influences also have other—and more positive—effects on the developing nervous system. Stimulus information—including objects to explore or manipulate, other organisms to interact with, and so on—seems by itself a spur to neural growth. In addition, the specifics of the stimulation help to refine the nervous system’s functioning—so that the visual system, for example, becomes especially sensitive to the specific shapes in the infant’s environment; circuits involved in language use are likewise adjusted so that the neural apparatus for understanding and producing language is pre-cisely tuned to the language being spoken in the child’s surroundings.
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