Marc H. Bornstein and Helen G. Bornstein
Child and Family Research
National Institute of Child Health and Human Development
Building 31 -- Room B2B15
9000 Rockville Pike
Bethesda, Maryland 20892
Infancy is the period of life between birth and the beginning of language, one-and-a-half to two years later. Indeed, the English word "infant" derives from the Latin in + fans, which means, literally, "non-speaker." This early stage of development is of substantial interest because of the dramatic developmental changes that take place in human beings in this short period of time. There are great advances in the shape and capacity of the body and its muscles, substantial growth in complexity of the nervous system, rapid development of sensory and perceptual ability, achievement of major milestones in understanding and mastering objects in the world, basic acquisition of language skills, formation of specific and central social bonds, and emergence of characteristic personal and social styles. At no other point in the lifespan do so many aspects of the individual change so rapidly and so fundamentally. From the newborn, unable to move from the position in which placed and unable to understand the simplest phrase, emerges the toddler, moving about at lightening speed and following complex speech commands.
In this chapter, we will take a close look at how the remarkable transformations of infancy occur and review what accomplishments are normally expected in this first phase of life after birth. We will examine the major milestones and achievements of this period of development in each of several spheres of growth: physical and motor development, perceptual development, cognitive development, language development, and social and emotional development. We will also look at environmental factors -- in the family, the community, and the culture -- which serve to foster or hamper development. But first we will briefly review the critical nine months in which development begins, the prenatal period; they set the stage for development of the total child in infancy and after.
There is one caveat to the body of data we rely upon for information about the child: much of it derives from studies of infants from middle class families living in Western, industrialized countries. Indeed, 95% of the world's children are unrepresented in these studies. Wherever possible, we refer to observations of infant development in other cultures; they serve to highlight the impact of culture over all facets of development.
Conventionally, the clock starts ticking on a person's age beginning at birth; but in fact development is already proceeding at a very rapid pace for the nine months preceding birth. With the joining of female egg and male sperm, the genetic makeup of the child is determined, and the interplay between organism and environment that is to characterize development throughout the life cycle begins. During the nine-month period of gestation, the child develops from a solitary cell into a complex organism consisting of billions of cells differentiated by form and function. In the first two weeks after conception, the fertilized cell divides rapidly and implants in the uterine wall. Differentiation of cells is now also underway to create the numerous organs and systems which eventually make human beings self-sufficient. By the third week, the head is distinguishable along with primitive circulatory and skeletal systems. At approximately one month, the heart begins to beat. By the end of the second month, the embryo is little more than one inch in length and weighs less than an ounce; but it already has a human form and all of its organs. Now called a fetus, the organism will experience great changes in size and in proportions. For example, the head goes from being one-half the total body length at two months to one-third the body length at three months to one-fourth the body length at four months at which point it stabilizes until birth. As the fetus matures, it begins to swallow, urinate, move, suck its thumb, and even hear, in utero. Indeed, studies show that fetuses learn: infants exposed to certain sounds in utero appear to remember and respond differentially to those stimuli after birth. Fetuses may also begin to display individual characteristics: some are more active than others, favor different positions and so forth. By the beginning of the third trimester, fetuses can survive outside the womb.
Although such an intricate and complex affair as the formation of human being could only take place under the direction of a fine-tuned genetic program, the environment is also a key player in the process. A number of environmental factors influence the course of development in utero, the most prominent being maternal diet, disease and exposure to environmental toxins and drugs. Maternal malnutrition is a subject of much concern today because it is so common in many regions of the world. Studies show that women who eat well and whose diets are rich in protein experience fewer complications during pregnancy and tend to have healthier babies. For example, how much protein a mother ingests during pregnancy has been associated with both the physical length of the infant at birth as well as with the infant's psychomotor development.
The deleterious effects of maternal drug use, alcohol consumption, and smoking are also now well documented as are the harmful effects of exposure to environmental toxins such as PCBs, DDT, mercury, and lead. But even subtler factors such as maternal stress during pregnancy can affect growth deleteriously. Stress can alter hormone levels which in turn can adversely influence prenatal development. At the moment, the most effective weapon at our disposal to combat these negative influences is education and good prenatal care.
During nine months gestation, life sustaining functions, such as supplying of oxygen and nutrients, the elimination of waste, and the regulation of body temperature, are taken care of by the mother. At the moment of birth, however, the baby must abruptly take on all these tasks him or herself. Although there are approximately -- births per minute world wide, it is no less remarkable each time how all systems come together to produce a living, breathing, sentient being who immediately begins to act and interact with the environment.
Newborns are well prepared to adapt to their new surroundings. At birth, they are in a high state of sensory preparedness. Moreover, they come equipped with a set of preprogrammed behaviors which are organized to guarantee their survival. For example, just minutes after birth, a baby put to the breast will root and then start sucking and swallowing. Rooting, sucking, and swallowing are all reflexes -- simple unlearned automatic responses to common stimuli that prepare the baby for extrauterine life. It is obviously quite adaptive that babies can perform this integrated set of behaviors without the need for experience or instruction, since it ensures the immediate ingestion of nutrients. Another group of reflexes comprising sneezing, coughing, blinking, and muscle withdrawal help the child avoid noxious or other unwanted stimuli. Still other reflexes may have had meaning at an earlier point in human development but continue now without obvious survival value. For example, the Moro reflex is the tendency babies have to swing their arms wide and bring them together again in the midline (as if around the body of a caregiver). This reflex is elicited by a loud sound, or when the baby suddenly falls or loses support. In nonhuman primates this movement, together with the grasping reflex (during the first week of life infant grasp is so strong that the infant is able to support its own weight) help the baby to secure itself to the mother's body.
As the infant matures, many reflexes will disappear and give way to more mature voluntary actions. For example, the grasping reflex, which newborns perform automatically when an object is placed in their palm, is replaced by intentional reaching and grasping at around three months of age. One explanation for this transition resides in the development of the brain's cerebral cortex. At birth, the subcortex of the brain which controls primitive reflex behavior is more developed than the cerebral cortex which is thought to be responsible for higher-level motor behavior and intellectual functioning. As the cerebral cortex develops, the reflexes drop out and more complex voluntary behavior takes over. Thus reflexes are not only adaptive, they are an index of normal development. The absence of a reflex which should be present, or the presence of one which should be absent, are early indications of neurological or other problems.
A number of tests have been developed to evaluate the physical well-being of newborns and young infants, and reflexes are one of the principal behaviors evaluated. In most American hospitals, about one minute and five minutes after birth, newborns are administered a test to determine if they are functioning normally and whether there is a need for intervention. This test is called the Apgar, named after Virginia Apgar (1953) its originator, and it rates babies as 0, 1, or 2 on each of five dimensions: heart rate, color, muscle tone, breathing effort, and reflex irritability. With 10 being the high score and zero the lowest, the majority of newborn babies score 7 or better on the Apgar; those who score 2 or lower are at a significant risk (Apgar and James, 1962; Self and Horowitz, 1980). Other human neonatal tests include the Dubowitz and the Neonatal Behavioral Assessment Scale, [which measure, indicate...?]
PHYSICAL GROWTH AND MOTOR DEVELOPMENT
Physical growth in the first two years of life is dramatic. An average baby will triple his or her birth weight in the first year and quadruple it by the end of the second. Moreover, the baby will achieve more than half of adult height by the age of two. The size of the brain also increases substantially, reaching approximately 75% of adult weight by the end of infancy. Most of the brain growth which occurs during infancy can be ascribed to myelinization and increases in the volume and density of dendritic connections. The rate of physical growth during infancy is greater than that of any subsequent period in the life span.
Observing and charting the course of physical growth in the child is important because normal physical development gives an indication of the well-being of the infant as a whole. Developing normal motor skills, for example, depends on a body which is growing in strength and proportion. Moreover, physical growth exemplifies certain principles of development which have significance for other psychological domains. First, development proceeds cephalocaudally; that is from head to toe. For example, the visual system reaches anatomical maturity earlier than do the legs; relatedly, babies achieve voluntary visual control much earlier than they learn to walk. Second, development proceeds proximodistally; that is from the center of the body outward. For example, the head and trunk of the body develop before the arms and legs, and babies learn to control their heads (neck muscles) before they learn to direct their limbs. Third, different systems in the body grow and develop at different rates. For example, if infants grew in height as rapidly as they did in weight, the average one-year-old would be approximately 5 feet tall. Finally, when we talk about growth and development generally, we tend to refer to norms. For example, the average American one-year-old weighs about 20 pounds and is about 30 inches long. But it is important to emphasize that while norms are handy descriptions, individuals vary greatly in the timing and course of their development.
Moreover, as we will see for all aspects of development, environmental factors play a significant role in the expression of individual variation. For example, at birth and throughout the first years of life, infants in more affluent countries tend to be healthier, heavier, and longer than their counterparts in less affluent countries. Moreover, within more affluent countries such as the United States, children born into low socioeconomic classes tend to grow more slowly than those from high socioeconomic classes, and often do not reach equivalent levels of height and weight (Eichorn, 1970; Tanner, 1978). Presumably, these differences reflect environmental factors such as prenatal and postnatal nutrition and care and maternal health and education.
It is clear that good nutrition in this period of rapid growth is important. The body needs a variety of nutrients to sustain normal growth and development. For example, protein is important for growth of tissue, carbohydrates supply the body with energy, and fat provides insulation. It is not just coincidence that human breast milk, which has evolved over time to match uniquely human needs, provides just the right balance of nutrients needed for healthy development in the first six months of life. Breast milk is higher in nutrients which promote brain growth and myelinization of nerve cells, important in the early months of human development, as compared to cow's milk which is higher in nutrients that promote tissue and muscle development, more important to cows who grow at almost twice the rate of humans. Moreover, the chemical composition of breast milk changes over time to suit the developing needs of pre-terms, newborns, and older infants.
Numerous studies now show that, especially in less developed countries, breastfed children are less likely to be malnourished than other children, and they are less likely to become ill, particularly with diarrhea and respiratory infections which are common in the developing world. This is especially true when colostrum (the breast fluid which is produced the first few days after birth) is given, and the child is exclusively breastfed for the first four to six months. If breastfed children do become ill, they are less likely to develop complications or die from diseases. The reasons for these beneficial effects are many.
Breast milk is sterile and ready to ingest. It requires no refrigeration, is not mixed with potentially contaminated water, and is not administered with potentially contaminated bottles or nipples; it therefore does not contain bacteria and viruses which cause infections. In addition, breast milk provides protection to the infant because it passes on immunological factors and other components produced by the mother to the infant. It is easier to digest, and especially during bouts of diarrhea provides nutrition in a readily absorbable form. Finally, even when mothers are malnourished, breast milk will reduce in volume more than in nutritional content. With proper nourishment, the brain and body of the baby grow, and as muscles and the nervous system mature, motor skills begin to emerge at a dramatic rate. With each new skill, babies attain greater mastery of their bodies and of their environment. What follows is a summary of the major motor milestones, divided into two parts, one for gross motor skills and one for fine motor muscle skills.
MILESTONES IN MOTOR DEVELOPMENT
Gross Motor Skills
1 mo. Lifts head while lying on stomach
2 mo. Lifts chest while lying on stomach
3 mo. Rolls over
4 mo. Sits with support
6-7 mo. Sits alone
8 mo. Stands with help
8 mo. Crawls, stands with help
11 mo. Stands alone
12 mo. Walks alone
17 mo. Walks up steps
23 mo. Jumps in place
Fine Motor Development
3 mo. Swats at dangling objects, grasps objects voluntarily (as opposed to by reflex)
4 mo. Visually guided reaching
5-6 mo. Lets go of objects, rotates wrist
8-9 mo. Thumb fully opposable, permitting pincer movement, combines objects at midline.
With the emergence of each new motor skill, a new world opens for the baby, allowing for the development of his or her perceptual, cognitive, and social skills as well. For example, once they can crawl, infants are no longer entirely dependent on caregivers to secure things they desire. This achievement can foster a sense of control over the environment. Crawling may also help to refine children's perception of depth, influence their fear of heights, and changes parents' attitude and treatment of their activities. Obviously, there is an intricate and organized pattern to this process or unfolding of infant skills and accomplishments.
However, motor development is not a function of physical maturation alone. Even in this sphere of life thought to be largely "biological," experiences also play a significant role. Infants today, for example, appear to master certain motor skills earlier than infants born fifty or a hundred years ago, and infants from different cultures appear to progress at different rates. Hopi infants begin to walk alone late (Dennis and Dennis, 1940); Balinese infants follow a different set of stages on their way to walking (Mead and McGregor, 1951); and Ganda and Wolof infants tend to be more advanced motorically (Ainsworth, 1967; Geber, 1956, 1958; Lusk and Lewis, 1972). Some data suggest that genetic differences among newborns may be responsible for some of these differences (e.g., the skeletons of native Africans may be more mature than those of Western Caucasians even at birth; Tanner, 1970). However, there is much data to support environmental effects as well. Hopi babies were traditionally constricted on cradle boards which may explain their locomotor retardation, while Balinese mothers carried their infants in a manner which Margaret Mead believed might promote the babies' unique motor performance. Babies in Ganda are raised in an atmosphere of physical freedom which may explain their accelerated development.
Kenyan Kipsigis babies have been found to be advanced in some motor skills (sitting, standing, and walking), but slower in others (head lifting, crawling, and turning over). A close look by researcher Charles Super at Kipsigis mothers revealed that 80 percent deliberately taught their infants to sit, stand, and walk. Super found that such practices were widespread among native Africans; indeed, he detected a substantial statistical association between the percentage of mothers who taught their babies to crawl and the average age of crawling in six African societies. Interestingly, Kipsigis infants who are reared like European babies tend to lose the advantage that their traditionally reared compatriots maintain, further suggesting that environmental factors can have a noticeable impact on psychomotor development (Super, 1976).
What does the world look, sound, smell, taste, and feel like to the normal infant? Is it all a "blooming, buzzing confusion" as a philosopher once proposed, or does the newborn come equipped with means ta make sense of his or her new world? Research tells us that infants have many capabilities which permit them from the start to appreciate the visual, auditory, chemical, and tactile stimuli in their environment in meaningful ways. We begin by considering infants' visual perception.
Even in the first few days after birth, babies appear to have relatively mature visual capabilities. For example, in the first hours of life, infants look selectively at parts of stimuli where there is information -- usually high-contrast features, such as the angle of a triangle -- instead of randomly scanning about. Neonates also track a slowly moving object with their eyes, and are sensitive to changes in brightness in their visual field. What is most important about visual behavior at this time is that newborns actively seek stimulation and input in ways which ensure that they will learn something about what they see. For example, they prefer novel stimuli to familiar ones, and they prefer patterned to solid colored forms.
But newborns also have visual deficits. For example, acuity is extremely poor at birth and the ability to scan is limited. However, these and other visual capabilities improve dramatically in the first months of life. For example, by one measure, acuity was found to increase from 20/800 at two weeks to approximately 20/70 at five-and-one-half months. Infants scanning ability also improves rapidly. Newborns tend to focus on a single feature of a form, which is usually a high contrast contour on the stimulus perimeter. By two months of age, infants are no longer captured by a single feature and can scan more of the perimeter of a shape and examine the internal features as well. Not surprisingly, it is also at about two months that infants begin to show a clear preference for the human face, related to their increased ability to scan the internal features of a stimulus. Also by two months (and perhaps younger), infants have largely normal color vision and can discriminate brightness almost as well as adults. By three months of age, babies recognize their own mothers' faces and can discriminate between some facial expressions.
By four months of age, infants not only appear to prefer some colors, but also to categorize the color world into discrete hues of blue, green, yellow, and red in way similar to adults. Sometime in the first half-year, infants show clear evidence that they are perceiving the elements of a complex pattern as part of a unified whole, rather than as separate elements. Depth perception seems to develop more gradually. Although some evidence of depth perception has been reported in infants two months and younger, it would appear to be quite limited at this age. It isn't until about seven months that children seem able to take advantage of all the different cues that signal depth (kinetic, pictorial, and binocular), and some argue that experience with independent locomotion, such as crawling, facilitates mature perception of depth. Lastly, infants begin to learn that even though objects move about in space -- approach, retreat, turn sideways -- those objects are still the same. This ability to perceive constancy in an ever-changing environment help infants to bring order to the perceptual world. By the time the child is one year of age, the visual world has come into focus, both literally and figuratively. Acuity has improved dramatically, and the basics of color, shape, depth, and orientation in space have been mastered. The other sensory modalities are also taking shape.
As mentioned earlier, the auditory system is functioning even in utero. Immediately after birth, the ears may be filled with fluid which temporarily impairs acuity. Thereafter, while the acuity of infants is not equal to that of adults, it is quite good and continuous developmental improvements take place during the first two years. Notably, infants seem to be specially "tuned" to human speech sounds. For example, very young infants will alter their behavior just to hear human speech. Moreover, babies as young as three days recognize their own mother's voices. Babies as young as one month of age distinguish among different speech sounds in an adult-like categorical manner; that is, babies can tell apart sounds like pah and bah, but not sounds equally different on a physical continuum but within the same speech category of sounds for adults, like two different sounding pahs. This suggests that infants may be "primed" in some ways to learn speech.
Infants may also be "primed" to distinguish among and even favor some tastes and smell. Neonates, just hours old, display characteristic facial expressions when sweet, sour, and bitter substances are placed on their tongues. A sweet stimulus evokes a relaxed expression, often accompanied by a slight smile and sucking. A sour stimulus evokes pursing the lips, wrinkling of the nose, and blinking the eyes. A bitter fluid evokes an expression of dislike, often followed by spitting or movements preparatory of vomiting. This preference for sweets is most probably an evolutionary adaption -- breast milk is sweet, many sour or bitter tasting substances are unhealthy or poisonous.
Newborns have also been found to respond in qualitatively different ways to different odors, favoring some and rejecting others. This ability, too, is put to good use by babies. Breast-fed infants, just days old, orient to gauze pads worn in the underarm area by their mothers (in preference to pads worn by other adults). Signature odors can therefore be used to help identify the mother. Newborns can also locate the source of odors and protect themselves from noxious smells by turning away from the source.
Newborns are also sensitive to touch. We know that gentle pats can sooth a fussy infant and that a needle prick to draw blood causes distress. We also know that sensitivity to touch continues to develop after birth. For example, the time it takes a baby to react to a needle prick decreases over the course of the first few months as a result of maturation of the nervous system.
Touch is an important source of information for infants. If you gently touch a newborn near the mouth, he or she will automatically turn its head in that direction in search of food. The warmth and pressure from contact with the mother's body also carry a message to young babies of security in moments of stress. And once the child develops the coordination necessary to bring objects to the mouth, touch works in concert with vision to inform infants about key properties of their new world. Infants as young as one month are influenced in which of two shapes to look at if one of the two matches a shape the babies had just explored orally. Indeed, observing a five-month-old in action, one comes away with the impression that in this stage of infancy both vision and reach are in the employ of oral exploration. An object is not fully appreciated until it has been mouthed.
Although the senses are functioning at a high level even from birth, experience as well as the lack of experience play important roles in the maintenance, attunement, and induction of perceptions in infancy. For example, certain experiences have been found to induce perception. One-month-olds who have a stabile hung over their cribs have been found to develop a visual defensive response to its sudden approach in half the time of a comparable group of babies without a stabile (Greenberg, Uzgiris, and Hunt, 1968). Similarly, it has been found that introducing an interesting stabile into the environment of institutionally deprived newborns nearly doubled their visually directed reaching and visual attentiveness (White and Held, 1966). Conversely, the lack of experience at a critical time may impair perception. For example, studies of children who developed strabismus at different ages, and who then had corrective surgery, found that children who developed the problem after three to four years of age, or who had corrective surgery within the first few years of life, were less likely to suffer permanent damage to the development of binocular vision, than children who were deprived of such experience in the first two years.
Speech provides an interesting example of a case where certain abilities infants have initially can be lost if not promoted by the language environment. For example, six-month-olds from English-speaking families discriminate pairs of Hindi speech contrasts (not used in English) as well as Hindi adults do, but English speakers four years of age and older do not (Werker, Gilbert, Humphrey and Tees, 1981; Werker and Tees, 1983). In a separate experiment with a different language group, the same researchers determined that the "loss" in perception already occurred by the end of the first year of life. Thus, the perceptual state of the newborn is highly developed, and perception in the older infant approximates mature, but as we will see in the next section how the infant interprets and makes sense of incoming information is longer in developing.
Newborns, and even fetuses, are capable of learning. For example, infants just two hours old can learn an association between gentle head strokes and the subsequent delivery of a sweet solution into their mouths. Compared with infants not so trained, neonates who had this training show more head-orienting and sucking responses. Furthermore, three-day-olds can learn to moderate the average interval between sucking bursts (either increase or decrease) to invoke their mothers' voice. Young infants will also increase their sucking to produce visual displays and music. However, not all associations are equally easy for infants to learn. To date, researchers have had the most success "teaching" associations which have some adaptive or biological significance to the child. For instance, gentle stroking may naturally accompany nursing and therefore infants may be prepared to learn an association between the two. The behavioral state of the child may also affect success on a learning task. Infants have been found to be most receptive to learning when they are in a period of quiet alertness, next most receptive during periods of distress, and least receptive when drowsy or asleep. Also, the economics of the response demanded of the infant is a factor. If the cost of the response in terms of energy expended exceeds the value of the reward for performing, the child will not necessarily perform the response. Finally, it should be noted that while even newborns demonstrate myriad learning skills in a laboratory setting, it is not clear to what degree they use these skills in daily life.
Habituation, the decline in attention to a stimulus that has been presented repeatedly or is available continuously, also indicates learning in infancy. It is theorized that habituation involves the construction of some sort of mental model or internal representation of a stimulus and the ongoing comparison of new stimulation with that representation (i.e., one learns about the stimulus and then compares incoming information with what has been learned). Habituation has also been found to occur in the newborn period.
Although children may be capable of learning from birth (and even before), their ability to do so develops throughout infancy. The nervous system is maturing rapidly in the first months after birth, and this allows for longer periods of alertness which in turn permits greater opportunity to learn. By six months of age, almost all the physiological immaturities which limited attention, perception, and information processing in infants have disappeared. As a result, between three and six months of age, infants' abilities to attend selectively to the environment and to encode what they perceive improve substantially, and these improvements are reflected in infants' performance on learning tasks. For example, it takes more than six times as many trials for a newborn to learn than for a five-month-old, and only one-and-one-half times as many trials for a three-month-old. Similarly, with the habituation paradigm, two- to three-month-olds required nearly five to six times as much exposure to a stimulus to achieve habituation as six- to seven-month olds. Thus, as infants mature, they also become more efficient learners.
Imitation is yet another way that infants demonstrate learning, although there is still substantial debate in the literature about the age at which children are capable of imitating. There is no debate, however, about how powerful imitation is as a learning tool once it does come on line. Imitation is generally believed to develop gradually and in an orderly fashion over the first two years. In the first six months of life, infants seem to be able to reproduce certain behaviors which they are already capable of producing in response to similar adult actions. So, if an adult coos, the baby may coo back. This is an action which a newborn tends to do spontaneously as well. In the second half of the first year, babies may try to venture into new territory. For instance, they are now making sounds, like babbling, which approximate the sounds of human speech, absent, of course, meaningful words. Indeed, the six-month-old's babbling may already resemble the parent language in pitch and contour (as we shall discuss in the Language section). At about a year, children start to imitate unfamiliar behaviors with accuracy. They will imitate language with obvious success and will become little copycats, following their siblings and parents around doing everything they see done, checking frequently to make sure they get it right. By eighteen months, they can imitate accurately, even in situations where they can't directly monitor their own behavior, as for example when imitating facial expressions. Once imitation is in full swing, it will become one of the major tools of learning in the early years.
By definition, all of the types of learning described involve the infant's remembering certain associations, whether they are associations between the infant's own actions and their environmental consequences or between two stimuli. In other words, learning includes memory as well. Even newborns are capable of remembering although their memory may last only a few minutes. By one month of age, however, babies are capable of remembering from one day to the next, and by the time they are two- to three- months-old, they may be able to remember some things for as long as one month, although perhaps with some prompting. In fact, memory continues to improve throughout infancy. For example, when twelve- to eighteen-month-olds are presented with memory tasks of increasing complexity, twelve-month-olds can remember and imitate only the simple actions correctly, whereas eighteen-month-olds execute all the tasks reasonably well. Memory improves substantially after infancy, as children gain general knowledge which gives their memories context, and as they learn to use strategies for remembering. In the area of cognition, it appears that as the children get older their ability to process information improves; they habituate to a new stimulus more rapidly, they can be conditioned to a wider range of stimuli, they can retain information for longer periods of time, and they can process more complex stimuli.
Learning, by definition, also involves an interaction with the environment -- because that is what children are learning about. But can some environments promote learning and others retard it? The answer is yes. For example, infants who have had experience with responsive or contingent stimulation (e.g., a mobile that can be activated by a child's kick) have been shown to learn a new, but similar task more quickly than children who have not had such experience. It is hypothesized that infants' experiences with contingent stimulation lead them to develop a sense of effectance. They learn not only to activate the mobile, but also the greater lesson that they can control aspects of their environment. They also may become more motivated to learn by their positive experiences with contingent stimuli. On the other hand, babies who have experienced primarily noncontingent stimulation, may develop a "learned helplessness" with regard to their environment and lose their motivation to master their surroundings. In a slightly different vein, research shows that different types of caregiving styles and home environments may have differential effects on infant performance on standardized intelligence tests, with responsivity and greater variety of stimulation being associated generally with higher test scores. Thus, environments which are stimulating and particularly those which contain responsive or contingent stimulation, can promote learning. Environments devoid of stimulation can retard it.
Another approach to cognitice development is that of Jean Piaget.
Piaget provides another view of cognition in childhood which attempts to explain how children come to understand the world. According to Piaget, children's understanding of the world, including their understanding of such basic concepts as time, space, causality, and the nature of objects, comes from acting on the world and not just from observing. In other words, children construct knowledge initially from their own actions. For example, the child takes some action (e.g., starts to suck an object in his mouth); this generates feedback (e.g., he or she discovers the object is a fist and not a nipple). If they were to suck a fist successfully, they would have to modify the sucking to accommodate the different shape and size. This adaptation on the part of the child, to accommodate the environment, brings the child to a new level of understanding. Piaget claimed that the child proceeds from one level of understanding to the next in an invariant sequence of stages which are age-related, starting with the sensorimotor period in infancy which is itself divided into six stages.
At the beginning of the sensorimotor period, children are what Piaget termed "egocentric". They see the world only from the perspective of their own actions and their effects on the world, and are unaware that objects in the environment exist outside themselves. Through chance discoveries, however -- the child kicks the crib, and a mobile shakes -- the child begins to learn about objects and their properties, and objects begin to take on an independent existence, thus beginning to repeat actions to cause effects intentionally. Then they begin to experiment with cause and effect. For example, if they notice that milk drips from a tilted bottle, they might systematically vary the angle and pressure on the bottle to see how fast the milk will drip. And, finally, after substantial experience with trial and error, they begin to be able to imagine in their minds what effects different actions will have without the need for direct experience. This is the dawn of insightful problem solving and the beginning of the end of sensorimotor egocentrism. The child now has a sufficient understanding of basic properties of objects that solutions to problems can be effected through thought instead of action. The child can now represent objects in his or her mind and manipulate them mentally. Over time, this ability will allow the infant to see things from other peoples' perspective. Although egocentrism is on the wane, it does not entirely disappear for quite some time.
Piaget's theory of cognitive growth posits invariant and hierarchical stage-like progression from infant sensorimotor behavior through formal logic in adulthood. He applied his theory to different spheres of development. For example, he posited that the child's development of the concept of objects as independent entities evolved gradually from out of sight, out of mind to I know it is still there because I saw you hide it to I know it is there because I can imagine that is where you hid it. By 24 months and with the attainment of mental representation, the infant has grasped the concept that objects have an independent, enduring existence.
Infant play is another sphere of development which fits nicely into the Piagetian scheme of development. Importantly, play follows an ordinal sequence of development which mirrors the decline in egocentrism and the rise of mental representation outlined in Piaget's six sensorimotor stages. Initially, play centers on actions generated by the child as opposed to objects (e.g., waving arms and kicking legs). Then children begin to manipulate single objects (e.g., banging a spoon on the table or shaking a rattle). The focus is initially more on the action than on the object. Undifferentiated exploratory play which is not specific to the object gives way to functional/relational play which is tailored to the object and may involve exploration of the relations between objects (putting a block in a box or a spoon in a cup). With the advent of mental representation comes pretend or symbolic play which also follows a set pattern. Pretend play develops from play that involves only the self (pretending to sleep), through pretense involving self-object relations (pretending to drink from a cup), to pretense involving objects exclusively (having a doll pretend to eat).
Several attempts have been made to predict later success on intelligence tests from measures of cognitive performance in infancy. Standard psychometrically-based infant examinations such as the Bayley Scales of Infant Development have failed to predict future test performance, as have conditioning measures. By contrast, habituation (and novelty responsiveness) in infancy have been shown to have a small but significant correlation with cognitive performance in later childhood. Tests based on Piagetian theory, like the Uzgiris-Hunt Ordinal Scales of Psychological Development have proved to have selective predictive validity. A milestone that "ends" infancy is the acquisition of language, but many aspects of infancy prepare the child for this triumph.
One of the most remarkable accomplishments of infancy is the acquisition of language. Any adult who has visited a foreign country with a language very different from his or her own will have some insight into the formidable task which is set for the newborn. Human language functions on at least three different but interrelated systems: sound, syntax, and semantics. Moreover, each level has two components, production and reception. First, sounds that are intended as language must be produced and perceived as different from other sounds; intonation, rhythm, and accent must be mastered in this process. Second is syntax, the rules according to which words are structured and arranged into meaningful sentences. Third is semantics, the use of words and phrases to represent real-world or imaginary referents. Consider the young child's task in deciphering a simple sentence such as: Mommybakesacake. The child must first segregate the sound stream into meaningful units, decode the grammatical structure linking the word forms, and match particular word forms with their real-world referents. Moreover, all this must take place instantaneously and simultaneously to be meaningful to the child. It is therefore quite startling that children still in diapers have not only cracked the language code, but are on the way to becoming rule-savvy, interactive conversationalists.
Because of the enormity of the task and the remarkable achievements of relatively immature beings, it has been suggested that infants do not approach this task with a clean slate, that there are an innate bases for language learning. For example, as mentioned earlier, infants appear to be primed to hear and respond to human speech. Moreover, there may be a universal sequence to the emergence of language (although timing of milestones may vary). Babies in different cultures still in the very beginning of life appear to be able to be able to perceive speech sounds in terms of categories (phonemes). Then they begin to coo (around two months) and then to babble (around four to six months) and then to produce their first words (at about one year). These one-word utterances frequently express complex ideas; they may refer to objects, express moods, or command actions. Context and vocal quality help to differentiate the child's intention.
In the middle of the second year, children tend to experience a word explosion. When their vocabularies reach somewhere between 50 and 100 words, also by about the middle of the second year, they begin to put words together -- more milk, baby sleep -- in what some have termed telegraphic speech. So-called operations of reference refer to the presence, recurrence, or nonexistence of objects in the child's world; operations of relations refer to actions taken on objects. Intonation may be added to this mix to indicate whether a declaration, interrogation, or imperative is intended. The fact that children from vastly different language communities follow a similar basic developmental trajectory in using grammatical operations of reference and relations gives considerable weight to a biological view of early language development. By the time that children reach 3+- word phrases, their grammars are virtually rule- governed.
What role does the environment play in this developmental process? Although some aspects of language development appear to follow a more or less fixed pattern early in life, experiences must be profound since children only learn the language of their culture. Thus, while babbling in different language groups shows no great difference in basic sound production repertoires, by the time infants are six months of age, phoneticians can differentiate the babbling of infants from one language group from another based on factors like pitch intonation. Likewise, while certain speech sounds appear to be discriminated in the first few months of life, independent of language group, by the end of the first year, children lose the ability to distinguish speech sounds not used in their language group.
Studies now reveal a multitude of ways in which the environment exerts its influence over early language acquisition. For example, on sound production, middle-class mothers in several cultures have been found to speak more frequently to their infants and to use a wider variety of sounds than working-class mothers. Not surprisingly, their babies have been found to produce more sounds in the first months of life than their working class counterparts (Field and Pawlby, 1980; Ninio, 1980; Snow, De Blauw, and von Roosmalen, 1979; Papousek, Papousek, and Bornstein, 1985). In a similar vein, infants who experience conversational or turn-taking types of interactions tend to produce a higher ratio of speech-like to nonspeech-like sounds than infants who experience random responsiveness.
Experience also influences aspects of early syntax. For example, one study showed that mothers who used more yes/no questions ("Can you dance?") have children who use more verb auxiliaries (like "can"). Interestingly, not all features of child language can be similarly influenced. In the same study, the number of noun and verb phrases per utterance were uncorrelated with any measured maternal speech variable, suggesting that there may be less room for variability and environmental influence with some aspects of language than others.
Finally, experience also plays a role in semantics (word-learning). For example, Americans, French, and Russians achieve the two-word-stage by one-and-a-half years of age, but certain other cultures do not do so until at least two years of age, arguably because they have less exposure to language (Slobin, 1972). Components of the mother-infant relationship also may play a significant role in language learning. For example, mothers who speak more and who encourage their four-month-old infants' attention more to the environment tend to have one-year-olds with larger productive vocabularies and four-year olds who score higher on a standardized IQ test, suggesting that maternal stimulation facilitates the development of infants' language skills. Lastly, single-word utterances used by mothers tend to appear first in their children's vocabularies.
A recurring question about the role of experience in language learning is whether there is a critical period for language development. Is there something special about infancy or early childhood which makes language learning possible or easier at that time? Studies of children who were deprived of language experience in early childhood on account of abuse or neglect have found that, while some of children eventually develop language skills, their competence never reaches normal levels despite, in some cases, years of training. This suggests that for learning a first language, there is something special about childhood. Interestingly, though, it is not the case for learning a second language. In one study which looked at the acquisition of Dutch by a group of English speakers ranging in age from three to adulthood, it was found that twelve to fifteen-year-olds learned the fastest and preschoolers learned the slowest.
Infants of the same age vary among themselves on nearly every index of language development. The most advanced infants may imitate a word by nine months, whereas the least advanced only first do so at eighteen months. At about one year in U.S. American babies, comprehension ranges from 10 to 75 words, and production from 0 to 45 words; at about two years, individual toddlers range from 8 to 434 words. Infants also show extreme stylistic differences. One kind of child is "referential" with an early vocabulary marked by a high proportion of object labels and speech organized around information; another is "expressive" with an early vocabulary comprised of relatively more pronouns and action words and speech marked by social formulae and routines intended to communicate feelings and desires. Clearly, these are extremes; most children exhibit a more balanced picture of referential and expressive speech which they presumably acquire by participating in a diversity of everyday object-and social-oriented interactions. Language is at base a social affair, and it is to the development of social features in the infant that we turn last.
What follows is a brief summary of milestones in language learning in the first two years. Milestones in comprehension and production are listed separately, since they proceed at different rates; virtually all researchers agree that comprehension precedes production developmentally.
MILESTONES OF LANGUAGE DEVELOPMENT
Birth Responds and attends to speaking voice
6 mo. Discriminates between friendly and angry speech
9 mo. Understands gestures like peek-a-boo and responds to bye-bye
10-12 mo. Responds to simple commands
13 mo. Understands 50 words
2 mo. One syllable
2-3 mo. Cooing
3-6 mo. Babbling
6 mo. Clear vocalization of several syllables
7-9 mo. Two syllables with repetition of first: "ma-ma, "da-da"
8-18 mo. Says first word
15-18 mo. Says five words or more
18 mo. Says 50 words
18-24 mo. Uses two words in combination
23-24 mo. Uses first pronoun, phrase, sentence
SOCIAL AND EMOTIONAL DEVELOPMENT
Humans are innately social creatures. Perhaps this is because at birth and for a long time thereafter, human infants are entirely dependent on others for care and sustenance. It is therefore not surprising that infants appear to be equipped from birth with abilities which help them to establish relationships with those around them. They are drawn to the sound of the human voice, the sight of a human face, and the smell of their mother. And they have a remarkable ability to communicate. For example, the cry which is present from birth, and the smile and the eye-to-eye gaze which appear shortly after, all send a message to the mother -- The cry: "I'm hungry," "I'm tired," "I just need to be held." The smile: "I like to be near you," "I like when you play." The gaze: "I like to hear you talk," and an averted gaze, "It's time for me to rest." These are just some of the many signals infants use to communicate with their caregivers. And parents have special ways of communicating with their children. They use touch and motion and gentle voices to soothe and calm; they seek eye-to-eye contact when they want to stimulate and play; they coo in response to their babies' coos; they simplify and modify speech and gestures and facial expressions to help the babies learn.
It is through this give and take or "conversation" between parent and child that children learn the rules of social conduct. Indeed, parents who respond promptly, reliably, and appropriately to their children's signals give them good messages from the start. They tell their children that they matter, that they can trust their parents to be there for them, and they give their children a sense of control. When babies cry, they are fed, held or changed by mother. Rather than feeling helpless, babies already begin to feel they can have an effect on the world and begin to develop a special relationship for the person or persons who care for them.
This special relationship is called attachment, and it develops in phases gradually over time. In the first phase, from birth to about eight weeks, babies signal to adults around them indiscriminately; they are happy to receive a response from anyone. They are, however, beginning to recognize their mother (her smell and her voice) and to associate her presence with the relief of distress, which leads to the second phase.
Between two and seven months, babies begin to prefer to interact with the person primarily responsible for their care; the person who feeds, holds, rocks, and plays with them. This is particularly rewarding to parents who now feel that they are special to the baby. The baby also begins to learn in this phase about some of the rules of social interaction. For example, in many Western cultures adult-infant play often consists of face-to-face games: Babies coo or smile or stick out their tongues and adults respond with similar actions. Babies come to expect their partners in play to take turns and respond with puzzlement or disappointment if they don't. They also learn to expect their caregivers to respond to their signals of distress or pleasure in a reliable and predictable manner. From these interactions, babies learn three things: (1) reciprocity -- partners take turns acting and reacting to each others behavior; (2) effectance -- babies' behavior can cause their caregivers to act; and (3) trust -- caregivers can be counted on to respond to baby's signal. Babies begin to develop a coherent view of the social world and concepts of themselves as individuals who significantly affect others. The degree to which children learn these rules probably depends in part on the nature of the caregiving they experience, with care that is affectionate, responsive, and attuned to the needs of the child producing better results.
In the third phase, which lasts from seven to twenty-four months, intentional social behavior becomes prominent. Infants now have some mobility and can seek out their caregivers. They tend to initiate interaction more and to protest when an attachment figure leaves. The onset of separation anxiety or protest signals a major cognitive as well as social milestone. It has been interpreted as a signal aimed at bringing the attachment figure back to the baby, and its emergence can be linked to the child's acquiring an understanding of the concept of person permanence -- the notion that people have a permanent existence outside of the infant. Indeed, theorists claim that until the child has the idea of person permanence and realizes that the caregiver exists even when he or she is not visible or audible to the child, the child is not capable of forming a true attachment. So, according to these theorists, Phase 3 marks the beginning of attachment. Wariness of strange adults also becomes evident in this period. Of course, though, there are major changes from the beginning to the end of this period; infants learn to tolerate growing distance from attachment figures, and they become more adept at interacting with peers and unfamiliar adults.
Social development by definition involves an interaction with the environment, in this case the environment of people. Not surprisingly, that environment in turn influences the course of the child's development of social relations. For instance, the degree to which caregivers are responsive in the early months of a child's life (the degree to which they respond to the infants' signals of distress, or the degree to which they reciprocate in play) may influence how much the child learns about reciprocity, effectance, and trust. This, in turn, may influence the security of infants' attachments to their caregivers or their ability to interact appropriately with others. For example, children reared in institutions with little opportunity to form attachments to one or a small group of individuals show multiple emotional problems throughout childhood. They tend to be perceived as too friendly and solicitous of adult attention and have difficulty forming relationships with peers. On the other hand, children of mothers who are responsive, and in tune with their children's needs, tend to have children who are securely attached later in childhood, and securely attached children have been found to be more cooperative, autonomous, popular, and socially competent than their insecurely attached peers.
Part and parcel of becoming a social being is gaining the ability to express and read emotions of others. Some emotions appear to be present from birth. For example, two-hour-old infants who were given sour, sweet, salty, and bitter substances to taste respond to the pleasant stimuli with facial expressions which adults clearly interpret as positive, and to unpleasant ones with expressions adults interpret as negative. There are two schools of thought on the development of emotions. One school proposes that emotions come on line over time, with interest, disgust, distress, and a precursor of surprise evident in newborns, anger, surprise, and joy emerging by three to four months, and fear and shyness emerging sometime in the second half-year of life. Another school proposes that infants may "feel" the whole range of basic emotions from birth and that only the ability to express those feelings develops over time. This view is supported by interviews with mothers of one-month-olds, the vast majority of whom believe that their babies express anger and fear, a percent surprise, interest, and joy, and only a few sadness.
Whether or not "feeling" of emotions develop, there is clearly progress in the manner in which emotions are expressed and the types of stimuli which elicit emotions. For instance, the smile develops over time from an involuntary reflex type behavior at birth, which is unrelated to external stimuli, to one which can be elicited by a variety of stimuli, including faces, bells, and bulls'-eyes, to the truly social smile for which the human face becomes the most provocative elicitor. By two months of age, a familiar face is the best stimulus of all. Laughter follows a similar progression. It first appears at around three or four months in response to active stimulation like kisses on the tummy or to certain sounds. Later, at around seven to nine months it can be elicited by less intrusive stimuli (a game of peek-a-boo). By twelve months, unusual or discrepant behavior (a parent sucking a bottle) or the anticipation of something funny (a parent making overtures to tickle) can trigger a laugh. Expressions of both anger and fear appear to rise dramatically at around seven to nine at a time when the child is beginning to become mobile.
All of these emotions appear to have adaptive value. For example, the smile is singularly effective in eliciting positive stimulation from a caregiver and prolonging interactions. What parent is not motivated to repeat an activity which has caused the child to smile? Fear, particularly of strangers and heights, emerges at a time when infants are just beginning to venture out on their own and helps to keep the baby close to the caregiver, who serves as a secure base for exploration and provides some defense from dangerous situations, such as stairs and precipices. It is interesting in this regard that the expressions of some of the basic emotions may be universal. Individuals from a wide range of cultures, including the United States, Japan, South American countries, and others, all interpret photographs of happiness, sadness, disgust, and anger in similar ways.
Another aspect of emotional development is the ability of infants to "read" or interpret the emotional expressions of others. This also appears to develop over time. In the first stage (from birth to one-and-a-half months), infants pay little attention to the internal features of faces and thus probably do not discriminate consistently among facial expressions. Between one-and-a-half to two months and four or five months, babies discriminate facial expressions and can even distinguish variations of a single expression although it is not clear that the emotional meaning of the expression is understood then. Only in the third stage (four or five to eight or nine months) is there clear evidence that emotional expressions are meaningful. In this stage, infants display emotional resonance, reacting with appropriate emotion to the emotional displays of others. In the fourth stage (around eight to nine months), infants begin to appreciate that others' messages may pertain to specific objects or events in the environment, and it is at this time that "social referencing" begins.
Social referencing entails the child's reference to the mother or other caregiver's emotional expressions to determine how the child should approach an uncertain situation (a new toy, an unfamiliar person, or a potentially dangerous situation like a visual cliff). Social referencing brings together a number of newly-developing skills in the infant and demonstrates the pervasiveness of emotions. Infants must be able to see and hear their caregivers, must be able to ascertain that their caregivers are attending to the environmental stimuli in question (a skill not cognitively possible before the children reach Piaget's stage 4 of sensorimotor development), must be able to learn the relation between the stimulus and the emotion for the future, and must have learned to trust their caregiver's judgment. But once they have learned these skills, the benefits are indisputable. It would be dangerous for infants to have to learn to avoid electrical outlets, noxious chemicals, and poisonous plants only by direct experience. Learning mediated by the emotional expressions of caregivers seems to be a biologically sensible alternative.
While the expression of emotions may be universal and human, the environment plays a decisive role in determining when and under what circumstances emotions should be expressed. Even in infancy, parents begin to transmit the social rules concerning when it is appropriate to laugh, cry, express anger, and so forth. However, it is not until after infancy that children are expected to be able to regulate their emotions successfully.
Socialization plays a key role in the development of emotions within cultures. For example, social smiling may develop earlier in children reared at home than in children reared in institutions, fear may develop earlier in abused infants than in other babies, and girls may be better than boys at understanding emotions because mothers typically show a wider range of emotions to girls than to boys. Cultural experiences also function to shape the expression of emotions: American and Japanese mothers, for example, have contrasting expectations for their infants, communicate those expectations, and behave in ways that promote independence and assertiveness of self-expression versus the acquisition of emotional identity and value from the group.
The development of the child as a social being is well under way by the end of infancy. The child is now a fully interactive partner within the family and ready to form relationships with peers. The child's strong emotional ties to family members, particularly parents, provides the security and confidence needed to forge other relationships and explore the world. Children have also learned from their caregivers some of the basic rules of social interaction such as turn-taking and how to read the emotions of others.
From the moment children are born, the process of molding them into functional members of their family, their community, and their culture begins. But it should not be forgotten that each child is an individual. One indicator of their individuality is their temperament, the characteristic style each person has in confronting the world around him. While everybody has moments when they are happy or sad or angry or anxious, the intensity and frequency of these feelings tend to vary dramatically among individuals. Each individual appears to have a characteristic, recurring set of emotional expressions which help to define the person -- calmness, anxiety, fearfulness, shyness, happiness, interest and so forth. Emotionality, along with other related characteristics, such as activity level, adaptability, distractibility, sociability, add up to the essence we call temperament.
Temperaments can significantly influence the course of a child's development. First, temperament affects how people respond to the child. An infant who frequently cries and is difficult to sooth may receive different responses from his or her parents than a child who frequently smiles and coos, and this may vary with culture. A very active or highly distractible child may have fewer opportunities to observe and learn than a less active, more persistent child.
However, temperament cannot be considered in a vacuum. Traits which are maladaptive in one setting, may be beneficial in another. Therefore, a good predictor of long-term adjustment of children is the "goodness of fit" between their temperament and their environment. A highly active, vivacious, but easily distractible child might have difficulty in a confined and busy environment, but might do quite well in an environment which provides outlets for his or her energy and opportunities for quiet concentration. Particular traits may not be good or bad in an absolute sense; outcome may depend instead on how sensitive caregivers are to children's needs. So parents should try to understand and find positive outlets for their children's temperament rather than criticize or fight their natural expression. Indeed, children themselves play a significant role in creating a complimentary environment. A quiet, sedentary child, for example, will more likely cry when confronted with rough and tumble play and smile when treated more gently, thereby discouraging the former and encouraging the latter and helping to mold the environment.
One of the important questions concerning infancy is whether the environment is somehow more critical to normal, healthy development in this initial phase of life than in other times during the lifespan. Do the patterns established at the beginning of life relating to motor skills, thought processes, language, and interpersonal relationships set the stage for later growth and stature? Alternatively, are innate trajectories so strong that they transcend environmental influences? Are the answers the same for all spheres of development?
We have yet to find definitive answers to these questions. It could be, for instance, that even in the case of severe language deprivation, some course of training as yet unknown could restore full language use to deprived or isolated children. However, from a societal point of view, we would prefer not to have to marshal extraordinary means to counteract past deprivations, even if it were possible. The more important issue from a societal point of view is to determine which environmental conditions are harmful to development and should therefore be avoided at the outset, and which are beneficial and should therefore be promoted. Of course in answering this question one must take into account cultural goals since an environment fostering independence may not, for example, be beneficial in a society which depends on uniformity of behavior. The multiplicity of cultures and possible desired outcomes make this task a difficult one, since almost all of the data collected to date use Western, industrialized cultural goals as a reference. At this point, then, we can only provide the data with a caveat as to its source and use it as a base for expanding our knowledge to the vast majority of children as yet unrepresented in the databank of infancy.
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