Conception occurs when the sperm from the male penetrates the cell wall of an egg from the female. Human development during the 38 weeks from conception to birth is divided into three phases. The first, the germinal period, lasts from the moment of conception until the time the fertilized egg is implanted in the wall of the uterus, a process that typically takes 10 to 14 days. A second phase, lasting from the second to the eighth week after conception, is called the embryonic period and is characterized by differentiation of the major organs. The last phase, from the eighth week until delivery, is called the fetal period and is characterized by dramatic growth in the size of the organism.

Prenatal development is extremely rapid; by the 18th day the embryo has already taken some shape and has established a longitudinal axis. By the ninth week the embryo is about 2.5 centimetres (one inch) long; face, mouth, eyes, and ears have begun to take on well-defined form, and arms, legs, hands, feet, and even fingers and toes have appeared. The sex organs, along with muscle and cartilage, also have begun to form. The internal organs have a definite shape and assume some primitive function. The fetal period (from about the second month until birth) is characterized by increased growth of the organism and by the gradual assumption of physical functions. By the 20th week the mother can often feel the movements of the fetus, which is now about 20 centimetres long. By the 32nd week the normal fetus is capable of breathing, sucking, and swallowing, and by the 36th week it can show a response to light and sound waves. The head of the fetus is unusually large in relation to other parts of its body because its brain develops more rapidly than do other organs. The seventh month is generally regarded as the earliest age at which a newborn can survive without medical assistance.

By definition, infancy is the period of life between birth and the acquisition of language approximately one to two years later. The average newborn infant weighs 3.4 kilograms (7.5 pounds) and is about 51 centimetres long; in general, boys are slightly larger and heavier than girls. (The period of the newborn covers the first five to seven days, which the infant normally spends recovering from the stresses of delivery.) During their first month, infants sleep for about 1618 hours a day, with five or six sleep periods alternating with a like number of shorter episodes of wakefulness. The total amount of time spent sleeping decreases dramatically, however, to 912 hours a day by age two years, and, with the cessation of nocturnal feedings and morning and afternoon naps, sleep becomes concentrated in one long nocturnal period. Newborns spend as much time in active sleep (during which rapid eye movements occur) as in quiet sleep, but by the third month they spend twice as much time in quiet as in active sleep, and this trend continues (at a much slower rate) into adulthood.

At birth the infant displays a set of inherited reflexes, some of which serve his very survival. An infant only two hours old typically will follow a moving light with his eyes and will blink or close them at the sudden appearance of a bright light or at a sharp, sudden sound nearby. The newborn infant will suck a nipple or almost any other object (e.g., a finger) inserted into his mouth or touching his lips. He will also turn his head toward a touch on the corner of his mouth or on his cheek; this reflex helps him contact the nipple so he can nurse. He will grasp a finger or other object that is placed in his palm. Reflexes that involve sucking and turning toward stimuli are intended to maintain sustenance, while those involving eye-closing or muscle withdrawal are intended to ward off danger. Some reflexes involving the limbs or digits vanish after four months of age; one example is the Babinski reflex, in which the infant bends his big toe upward and spreads his small toes when the outer edge of the sole of his foot is stroked.

The newborn baby can turn his head and eyes toward and away from visual and auditory stimuli, signaling interest and alarm, respectively. Smiling during infancy changes its meaning over the first year. The smiles that newborns display during their first weeks constitute what is called reflex smiling and usually occur without reference to any external source or stimulus, including other people. By two months, however, infants smile most readily in response to the sound of human voices, and by the third or fourth month they smile easily at the sight of a human face, especially one talking to or smiling at the infant. This social smiling, as it is called, marks the beginning of the infants emotional responses to other people.

Research shows the achievement of extraordinary perceptual sophistication over the first months of life. The fetus is already sensitive to stimulation of its skin, especially in the area around the mouth, by the eighth week of intrauterine development. Judging from their facial expressions when different substances are placed on their tongues, newborn infants apparently discriminate between bitter, salty, or sweet tastes; they have an innate preference for sweet tastes and even prefer a sucrose solution to milk. Newborns can also discriminate between different odours or smells; six-day-old infants can tell the smell of their mothers breast from that of another mother.

Much more is known, however, about infants ability to see and hear than about their senses of touch, smell, or taste. During the first half-year of life outside the womb, there is rapid development of visual acuity, from 20/800 vision (in Snellen notation) among two-week-olds to 20/70 vision in five-month-olds to 20/20 vision at five years. Even newborn infants are sensitive to visual stimulation and attend selectively to certain visual patterns; they will track moving stimuli with their gaze and can discriminate among lights that vary in brightness. They show a noticeable predilection for the sight of the human face, and by the first or second month they are able to discriminate between different faces by attending to the internal featureseyes, nose, and mouth. By the third month, infants can identify their mothers by sight and can discriminate between some facial expressions. By the seventh month, they can recognize a particular person from different perspectivesfor example, a full face versus a profile of that face. Infants can identify the same facial expression on the faces of different people and can distinguish male from female faces.

Newborns can also hear and are sensitive to the location of a sound source as well as to differences in the frequency of the sound wave. They also discriminate between louder and softer sounds, as indicated by the startle reflex and by rises in heart rate. Newborns can also discriminate among sounds of higher or lower pitch. Continuous rather than intermittent sounds and low tones rather than high-pitched ones are apparently those most soothing to infants.

Even young infants show a striking sensitivity to the tones, rhythmic flow, and individual sounds that together make up human speech. A young infant can make subtle discriminations among phonemes, which are the basic sounds of language, and is able to tell the difference between pa, ga, and ba. Furthermore, infants less than one year old can make discriminations between phonemes that some adults cannot because the particular discrimination is not present in the adult language. A distinction between ra and la does not exist in the Japanese language, and hence Japanese adults fail to make that discrimination. Japanese infants under nine months can discriminate between these two phonemes but lose that ability after one year because the language they hear does not require that discrimination.

Both movement and contrasts between dark and light tend to attract an infants attention. When an alert newborn is placed in a dark room, he opens his eyes and looks around for edges. If he is shown a thick black bar on a white background, his eyes dart to the bars contour and hover near it, rather than wander randomly across the visual field. Certain other visual qualities engage the infants attention more effectively than do others. The colour red is more attractive than others, for example, and objects characterized by curvilinearity and symmetry hold the infants attention longer than do ones with straight lines and asymmetric patterns. Sounds having the pitch and timbre of the human voice are more attractive than most others; the newborn is particularly responsive to the tones of a mothers voice, as well as to sounds with a great deal of variety. These classes of stimuli tend to elicit the most prolonged attention during the first 8 to 10 weeks of life. During the infants third month a second principle, called the discrepancy principle, begins to assume precedence. According to this principle, the infant is most likely to attend to those events that are moderately different from those he has been exposed to in the past. For instance, by the third month, the infant has developed an internal representation of the faces of the people who care for him. Hence, a slightly distorted facee.g., a mask with the eyes misplacedwill provoke more sustained attention than will a normal face or an object the infant has never seen before. This discrepancy principle operates in other sensory modalities as well.

Even infants less than one year old are capable of what appears to be complex perceptual judgments. They can estimate the distance of an object from their body, for example. If an infant is shown a rattle and hears its distinctive sound and the room is then darkened, the infant will reach for the rattle if the sound indicates that the object can be grasped but will not reach if the sound indicates that it is beyond his grasp.

More dramatically, infants will also reach for an object with a posture appropriate to its shape. If an infant sees a round object in the shape of a wheel and hears its distinctive sound and also sees a smaller rattle and hears its sound, he will reach in the dark with one hand in a grasping movement if he hears the sound of the rattle but will reach with both hands spread apart if he hears the sound associated with the wheel.

The four-month-old infant is also capable of rapidly learning to anticipate where a particular event will occur. After less than a minute of exposure to different scenes that alternate on the right and left side of their visual field, infants will anticipate that a picture is about to appear on the right side and will move their eyes to the right before the picture actually appears. Similarly, infants only five to six months old can detect the relation between the shape of a persons mouth and the sound that is uttered. Thus, they will look longer at a face that matches the sound they are hearing than at one where there is a mismatch between the mouths movements and the sound being uttered.

Infants develop an avoidance reaction to the appearance of depth by the age of 8 to 10 months, when they begin to crawl. This discovery was made on the surface of an apparatus called the visual cliff. The latter is a table divided into two halves, with its entire top covered by glass. One half of the top has a checkerboard pattern lying immediately underneath the glass; the other half is transparent and reveals a sharp drop of a metre or so, at the bottom of which is the same checkerboard pattern. The infant is placed on a board on the centre of the table. The mother stands across the table and tries to tempt her baby to cross the glass on either the shallow or the deep side. Infants younger than seven months will unhesitatingly crawl to the mother across the deep side, but infants older than eight months avoid the deep side and refuse to cross it. The crying and anxiety that eight-month-olds display when confronted with the need to cross the deep side are the result of their ability to perceive depth but also, and more importantly, their ability to recognize the discrepancy of sitting on a solid surface while nevertheless seeing the visual bottom some distance below. Both nervous-system maturation and experience contribute to this particular cognitive advance.

Finally, infants create perceptual categories by which to organize experience, a category being defined as a representation of the dimensions or qualities shared by a set of similar but not identical events. Infants will treat the different colours of the spectrum, for example, according to the same categories that adults recognize. Thus, they show greater attentiveness when a shade of red changes to yellow than when a light shade of red merely replaces a darker shade of the same colour. Five-month-old infants can tell the difference between the moving pattern of lights that corresponds to a person walking and a randomly moving version of the same number of lights, suggesting that they have acquired a category for the appearance of a person walking. By one year of age, infants apparently possess categories for people, edible food, household furniture, and animals. Finally, infants seem to show the capacity for cross-modal perceptioni.e., they can recognize an object in one sensory modality that they have previously perceived only in another. For example, if an infant sucks a nubby pacifier without being able to see it and then is shown that pacifier alongside a smooth one, the infants longer look at the nubby pacifier suggests that he recognizes it, even though he previously experienced only its tactile qualities.

Infants make robust advances in both recognition memory and recall memory during their first year. In recognition memory, the infant is able to recognize a particular object he has seen a short time earlier (and hence will look at a new object rather than the older one if both are present side by side). Although newborns cannot remember objects seen more than a minute or two previously, their memory improves fairly rapidly over the first four or five months of life. By one month they are capable of remembering an object they saw 24 hours earlier, and by one year they can recognize an object they saw several days earlier. Three-month-old infants can remember an instrumental response, such as kicking the foot to produce a swinging motion in a toy, that they learned two weeks earlier, but they respond more readily if their memory is strengthened by repeated performances of the action.

By contrast, recall memory involves remembering (retrieving the representation, or mental image) an event or object that is not currently present. A major advance in recall memory occurs between the 8th and 12th months and underlies the childs acquisition of what Piaget called the idea of the permanent object. This advance becomes apparent when an infant watches an adult hide an object under a cloth and must wait a short period of time before being allowed to reach for it. A six-month-old will not reach under the cloth for the hidden object, presumably because he has forgotten that the object was placed there. A one-year-old, however, will reach for the object even after a 30-second delay period, presumably because he is able to remember its being hidden in the first place. These improvements in recall memory arise from the maturation of circuits linking various parts of the brain together. The improvements enable the infant to relate an event in his environment to a similar event in the past. As a result, he begins to anticipate his mothers positive reaction when the two are in close face-to-face interaction, and he behaves as if inviting her to respond. The infant may also develop new fears, such as those of objects, people, or situations with which he is unfamiliari.e., which he cannot relate to past experiences using recall memory.

As stated previously, Piaget identified the first phase of mental development as the sensorimotor stage (birth to two years). This stage is marked by the childs acquisition of various sensorimotor schemes, which may be defined as mental representations of motor actions that are used to obtain a goal; such actions include sucking, grasping, banging, kicking, and throwing. The sensorimotor stage, in turn, was differentiated by Piaget into six subphases, the first four of which are achieved during the initial year. During the first subphase, which lasts one month, the newborns automatic reflexes become more efficient. In the second subphase, the infants reflex movements become more coordinated, though they still consist largely of simple acts (called primary circular actions) that are repeated for their own sake (e.g., sucking, opening and closing the fists, and fingering a blanket) and do not reflect any conscious intent or purpose on the infants part. During the third phase, lasting from the 4th to the 8th month, the infant begins to repeat actions that produce interesting effects; for example, he may kick his legs to produce a swinging motion in a toy. In the fourth subphase, from the 8th to the 12th month, the child begins coordinating his actions to attain an external goal; he thus begins solving simple problems, building on actions he has mastered previously. For example, he may purposely knock down a pillow to obtain a toy hidden behind it. During the fifth subphase, covering the 12th to 18th months, the child begins to invent new sensorimotor schemes in a form of trial-and-error experimentation. He may change his actions toward the same object or try out new ones to achieve a particular goal. For example, if he finds that his arm alone is not long enough, he may use a stick to retrieve a ball that rolled beneath a couch. In the final subphase of infancy, which is achieved by about the 18th month, the child starts trying to solve problems by mentally imagining certain events and outcomes rather than by simple physical trial-and-error experimentation.

The childs actions thus far have shown progressively greater intentionality, and he has developed a primitive form of representation, which Piaget defined as a kind of mental imagery that can be used to solve a problem or attain a goal for which the child has no habitual, available action. An important part of the childs progress in his first year is his acquisition of what Piaget calls the idea of object permanencei.e., the ability to treat objects as permanent entities. According to Piaget, the infant gradually learns that objects continue to exist even when they are no longer in view. Children younger than six months do not behave as if objects that are moved out of sight continue to exist; they may grab for objects they see but lose all interest once the objects are withdrawn from sight. However, infants of nine months or older do reach for objects hidden from view if they have watched them being hidden. Children aged 12 to 18 months may even search for objects that they have not themselves witnessed being hidden, indicating that they are capable of inferring those objects location. Show such a child a toy placed in a box, put both under a cover, and then remove the box; the child will search under the cover as though he inferred the location of the toy.

The first of the two basic sounds made by infants includes all those related to crying; these are present even at birth. A second category, described as cooing, emerges at about eight weeks and includes sounds that progress to babbling and ultimately become part of meaningful speech. Almost all children make babbling sounds during infancy, and no relationship has been established between the amount of babbling during the first six months and the amount or quality of speech produced by a child at age two. Vocalization in the young infant often accompanies motor activity and usually occurs when the child appears excited by something he sees or hears. Environmental influences ordinarily do not begin to influence vocalization seriously before two months of age; in fact, during the first two months of postnatal life, the vocalizations of deaf children born to deaf parents are indistinguishable from those of infants born to hearing parents. Environmental effects on the variety and frequency of the infants sounds become more evident after roughly eight weeks of age. The use of meaningful words differs from simple babbling in that speech primarily helps to obtain goals, rather than simply reflecting excitement.

Emotions are distinct feelings or qualities of consciousness, such as joy or sadness, that reflect the personal significance of emotion-arousing events. The major types of emotions include fear, sadness, anger, surprise, excitement, guilt, shame, disgust, interest, and happiness. These emotions develop in an orderly sequence over the course of infancy and childhood.

Even during the first three or four months of life, infants display behavioral reactions suggestive of emotional states. These reactions are indicated by changes in facial expression, motor activity, and heart rate and of course by smiling and crying. Infants show a quieting of motor activity and a decrease in heart rate in response to an unexpected event, a combination that implies the emotion of surprise. A second behavioral profile, expressed by increased movement, closing of the eyes, an increase in heart rate, and crying, usually arises in response to hunger or discomfort and is a distress response to physical privation. A third set of reactions includes decreased muscle tone and closing of the eyes after feeding, which may be termed relaxation. A fourth pattern, characterized by increased movement of the arms and legs, smiling, and excited babbling, occurs in response to moderately familiar events or social interaction and may be termed excitement. In the period from 4 to 10 months, new emotional states appear. The crying and resistance infants display at the withdrawal of a favourite toy or at the interruption of an interesting activity can be termed anger. One-year-old infants are capable of displaying sadness in response to the prolonged absence of a parent.

Finally, infants begin displaying signs of the emotion of fear by their fourth to sixth month; a fearful response to noveltyi.e., to events that are moderately discrepant from the infants knowledgecan be observed as early as four months. If an infant at that age hears a voice speaking sentences but there is no face present, he may show a fearful facial expression and begin to cry. By 7 to 10 months of age, an infant may cry when approached by an unfamiliar person, a phenomenon called stranger anxiety. A month or two later the infant may cry when his mother leaves him in an unfamiliar place; this phenomenon is called separation anxiety. It is no accident that both stranger and separation anxiety first appear about the time the child becomes able to recall past events. If an infant is unable to remember that his mother had been present after she leaves the room, he will experience no feeling of unfamiliarity when she is gone. However, if he is able to recall the mothers prior presence and cannot understand why she is no longer with him, that discrepancy can lead to anxiety. Thus, the appearance of stranger and separation anxiety are dependent on the improvement in memorial ability.

These emotions in young infants may not be identical to similar emotional states that occur in older children or adolescents, who experience complex cognitions in concert with emotion; these are missing in the young infant. The older childs anger, for example, can remain strong for a longer period of time because the child can think about the target of his anger. Thus, it may be an error to attribute to the young infant the same emotional states that one can assume are present in older children.

Perhaps the central accomplishment in personality development during the first years of life is the establishment of specific and enduring emotional bonds, or attachment. The person to whom an infant becomes emotionally attached is termed the target of attachment. Targets of attachment are usually those persons who respond most consistently, predictably, and appropriately to the babys signals, primarily the mother but also the father and eventually others. Infants are biologically predisposed to form attachments with adults, and these attachments in turn form the basis for healthy emotional and social development throughout childhood. Infants depend on their targets of attachment not only for food, water, warmth, and relief from pain or discomfort but also for such emotional qualities as soothing and placating, play, consolation, and information about the world around them. Moreover, it is through the reciprocal interactions between child and parent that infants learn that their behaviour can affect the behaviour of others in consistent and predictable ways and that others can be counted on to respond when signaled.

Infants who do not have a particular adult devoted to their care often do not become strongly attached to any one adult and are less socially responsiveless likely to smile, vocalize, laugh, or approach adults. Such behaviour has been observed in children raised in relatively impersonal institutional surroundings and is shared by monkeys reared in isolation.

The social smiling of two-month-old infants invites adults to interact with them; all normal human infants show a social smile, which is, in fact, their first true sign of social responsiveness. The social smile is apparently innate in the human species. At about six months of age infants begin to respond socially to particular people who become the targets of attachment. Although all infants develop some form of attachment to their caregivers, the strength and quality of that attachment depends partly on the parents behaviour to the child. The sheer amount of time spent with a child counts for less than the quality of the adult-child interaction in this regard. The parents satisfaction of the infants physical needs is an important factor in their interaction, but sensitivity to the childs needs and wishes, along with the provision of emotional warmth, supportiveness, and gentleness are equally important. Interestingly, mothers and fathers have been observed to behave differently with their infants and young children: mothers hold, comfort, and calm their babies in predictable and rhythmic ways, whereas fathers play and excite in unpredictable and less rhythmic ways.

One significant difference has been detected in the quality of infants attachment to their caregiversthat between infants who are securely attached and those who are insecurely attached. Infants with a secure attachment to a parent are less afraid of challenge and unfamiliarity than are those with an insecure attachment.

During the first two years of life, the presence of targets of attachment tends to mute infants feelings of fear in unfamiliar situations. A one-year-old in an unfamiliar room is much less likely to cry if his mother is present than if she is not. A one-year-old is also much less likely to cry at an unexpected sound or an unfamiliar object if his mother is nearby. Monkeys, too, show less fear of the unfamiliar when they are with their mothers. This behavioral fact has been used to develop a series of experimental situations thought to be useful in distinguishing securely from insecurely attached infants. These procedures consist of exposing a one-year-old to what is known as the strange situation. Two episodes that are part of a longer series in this procedure involve leaving the infant with a stranger and leaving the infant alone in an unfamiliar room. Children who show only moderate distress when the mother leaves, seek her upon her return, and are easily comforted by her are assumed to be securely attached. Children who do not become upset when the mother leaves, play contentedly while she is gone, and seem to ignore her when she returns are termed insecurely attachedavoidant. Finally, children who become extremely upset when the mother leaves, resist her soothing when she returns, and are difficult to calm down are termed insecurely attachedresistant. About 65 percent of all American children tested are classed as securely attached, 21 percent as insecurely attachedavoidant, and 14 percent as insecurely attachedresistant. All other things being equal, it is believed that those children who demonstrate a secure attachment during the first two years of life are likely to remain more emotionally secure and be more socially outgoing later in childhood than those who are insecurely attached. But insecurely attachedresistant children are more likely to display social or emotional problems later in childhood. The development of a secure or insecure attachment is partly a function of the predictability and emotional sensitivity of an infants caregiver and partly the product of the infants innate temperament.

Individual infants tend to vary in their basic mood and in their typical responses to situations and events involving challenge, restraint, and unfamiliarity. Infants may differ in such qualities as fearfulness, irritability, fussiness, attention span, sensitivity to stimuli, vigour of response, activity level, and readiness to adapt to new events. These constitutional differences help make up what is called a childs temperament. It is believed that many temperament qualities are mediated by inherited differences in the neurochemistry of the brain.

Most individual differences in temperament observed in infants up to 12 months in age do not endure over time and are not predictive of later behaviour. One temperamental trait that is more lasting, however, is that of inhibition to the unfamiliar. Inhibited children, who account for 1020 percent of all one-year-old children, tend to be shy, timid, and restrained when encountering unfamiliar people, objects, or situations. As young infants, they show high levels of motor activity and fretfulness in response to stimulation. (They are also likely to be classified as insecurely attachedresistant when observed in the strange situation.) By contrast, uninhibited children, who account for about 30 percent of all children, tend to be very sociable, fearless, and emotionally spontaneous in unfamiliar situations. As infants, they display low levels of motor activity and irritability in response to unfamiliar stimuli. Inhibited children have a more reactive sympathetic nervous system than do uninhibited children. Inhibited children show larger increases in heart rate in response to challenges and larger increases in diastolic blood pressure when they change from a sitting to a standing posture. In addition, inhibited children show greater activation of the frontal cortex on the right side of the brain, while uninhibited children show greater activation of the frontal cortex on the left side.

These two temperament profiles are moderately stable from the second to the eighth year; studies reveal that about one-half of those children classed as inhibited at age two are still shy, introverted, and emotionally restrained at age eight, while about three-quarters of those children classed as uninhibited have remained outgoing, sociable, and emotionally spontaneous.

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