HUMAN BEHAVIOUR

the expressed and potential capacity for activity in the physical, mental, and social spheres of human life. Human behaviour is a function of a person's cognitive, emotional, and social capabilities, which grow or change over the entire course of the life span, from infancy and childhood on into adolescence and then adulthood. Just as the human body undergoes change continuously from infancy through adulthood, so human behaviour never reaches a fixed level of development. Behavioral development, in fact, is closely tied to bodily development, and the generally recognized states in the two coincide to a large degree. The field of human development, or developmental psychology, attempts to describe and explain the changes in human behaviour over the life span. Most researchers have concentrated on the period from birth through adolescence, owing to both the rapidity and magnitude of the changes observed during those phases and to the fact that they culminate in the optimum mental functioning of adulthood. But while the study of the development of human behaviour is sometimes restricted to the so-called formative years, it more properly treats behaviour as developing over the full course of the life cycle. Basic philosophical disputes over the fundamental nature of children and their mental growth occupied psychologists throughout much of the 20th century. The most important of these disputes concerned the relative importance of internal bodily or genetic determinants of behaviour and external, environmental influences. The prevailing approach posits the combined influence of nature and nurture, whereby individuals are genetically predisposed toward certain behaviours that are to varying degrees subject to shaping by external factors. The advances in cognition, emotion, and behaviour that occur universally among people at certain points in the life span require both maturation (i.e., genetically driven growth of the central nervous system) and events, experiences, and influences in the physical and social environment. Generally, the physical maturation of the nervous system cannot cause a psychological function to emerge; it does, however, permit such a function to occur and sets limits on its earliest time of appearance. Three theories of human development have been dominant during the 20th century, each addressing different aspects of psychological growth. The Austrian psychoanalyst Sigmund Freud explained personality and emotional development in childhood as being affected by both biological and psychosocial forces operating within the family. Freud's model of personality structure has proven particularly influential. According to him, a wholly unconscious mental structure called the id contains a person's inborn, instinctual drives (e.g., sex, affection, aggression, and self-preservation), and the reality-oriented portion of the personality, the ego, develops in infancy and childhood and uses a variety of conscious and unconscious mental processes to try to satisfy id instincts while also attempting to maintain the individual in relation to the environment. The development of behaviour in childhood therefore predominantly consists of the emergence of the functions of the ego, which is responsible for channeling the discharge of fundamental drives and for controlling perceptual and intellectual functions in the process of negotiating realistically with the outside world. Freud's concept of unconscious motivations and mechanisms has proven highly influential, especially in psychiatry, and his concentration on emotional development in childhood influenced even those schools of thought that rejected his own psychoanalytic theories. The Swiss psychologist Jean Piaget took the intellectual functioning of adults as the central phenomenon to be explained and tried to trace how people acquire the ability to think logically and draw valid conclusions about the world from evidence. One of his fundamental assumptions is that early intellectual growth arises primarily out of the child's physical interactions with objects in his environment. As they grow, children organize and adapt their experiences with objects into increasingly sophisticated cognitive models that enable them to deal with future situations in more effective ways. Piaget traced the growth of children's logical understanding of the world around them in surprising detail, showing how they progress from simple reflex acts to consciously intended movements, to trial-and-error experiments with objects, and then to manipulations of the environment using mental models of probable events and outcomes. Piaget traced the child's successive mastery of such mental models as images, symbols, concepts or categories, and propositions to perform progressively more abstract mental operations in the course of rational and systematic thinking. Piaget's theories and observations provide an extremely useful framework for understanding cognitive and intellectual development from infancy to adulthood. A more distinctively American view focuses on the child's actions, rather than on his emotions or thinking, in the attempt to understand the acquisition of behaviour, motives, and values among children. Learning theory's major principles stress the effects of reward and punishment (as administered by parents, teachers, and peers) on the child's tendency to adopt the behaviour and values of others. Learning is defined as any relatively permanent change in behaviour that results from past experience. Learning theory invokes two widely recognized learning processes, classical and instrumental conditioning, to create or shape behavioral responses. In classical conditioning, a close tempered relation is maintained between pairs of stimuli in order to create an association between the two. In instrumental conditioning, rewards, such as praise and approval from parents, act as positive reinforcers of specific learned behaviours, while punishments act as negative reinforcers and make the future performance of such behaviours less likely to occur. Scientists who believe in the importance of conditioning and reinforcement use these mechanisms to explain the changing behaviour of children over the course of development. In retrospect, these and other theories seem to be neither logically rigorous nor able to account for both intellectual and emotional growth within the same framework. Research has thus tended to be descriptive, since the field still lacks a tight net of interlocking theoretical propositions that can comprehensively explain the development of human behaviour. See also psychological development. the potential and expressed capacity for physical, mental, and social activity during the phases of human life. Human beings, like other animal species, have a typical life course that consists of successive phases of growth, each of which is characterized by a distinct set of physical, physiological, and behavioral features. These phases are prenatal life, infancy, childhood, adolescence, and adulthood (including old age). Human development, or developmental psychology, is a field of study that attempts to describe and explain the changes in human cognitive, emotional, and behavioral capabilities and functioning over the entire life span, from the fetus to old age. Most scientific research on human development has concentrated on the period from birth through early adolescence, owing to both the rapidity and magnitude of the psychological changes observed during those phases and to the fact that they culminate in the optimum mental functioning of early adulthood. A primary motivation of many investigators in the field has been to determine how the culminating mental abilities of adulthood were reached during the preceding phases. This essay will concentrate, therefore, on human development during the first 12 years of life. This article discusses the development of human behaviour. For treatment of biological development, see human development. For further treatment of particular facets of behavioral development, see emotion; learning theory; motivation; perception; personality; and sexual behaviour, human. Various disorders with significant behavioral manifestations are discussed in mental disorder. Additional reading Authoritative texts on all stages of human development include Marc H. Bornstein and Michael E. Lamb (eds.), Developmental Psychology: An Advanced Textbook, 3rd ed. (1992); Urie Bronfenbrenner, The Ecology of Human Development: Experiments by Nature and Design (1979); Richard M. Lerner, Concepts and Theories of Human Development, 2nd ed. (1986); Richard M. Lerner and David F. Hultsch, Human Development: A Life-Span Perspective (1983); Daniel J. Levinson et al., The Seasons of a Man's Life (1978, reissued 1985); Lawrence Kohlberg, Essays on Moral Development, 2 vol. (198184); and Robert Plomin, Development, Genetics, and Psychology (1986).The major theories of human development discussed in the article are presented in Sigmund Freud, An Outline of Psycho-analysis (1949, reissued 1989; originally published in German, 1940); Erik H. Erikson, Childhood and Society, 2nd ed. rev. and enlarged (1964, reissued 1985); and Jean Piaget, The Origins of Intelligence in Children (1952, reissued 1974; also published as The Origin of Intelligence in the Child, 1953, reprinted 1977; originally published in French, 1936).Aspects of development in the infant, child, and adolescent are presented in Alan F. Guttmacher, Pregnancy, Birth, and Family Planning, rev. and brought up to date by Irwin H. Kaiser (1986); Daphne Maurer and Charles Maurer, The World of the Newborn (1988), a highly readable account of what psychologists have learned about the prenatal and early postnatal periods; Judy F. Rosenblith, In the Beginning: Development from Conception to Age Two, 2nd ed. (1992), an excellent summary of basic research on the infant; Jerome Kagan, The Nature of the Child (1984), a collection of essays on child development; Jane B. Brooks, The Process of Parenting, 3rd ed. (1991), a practical guide to child-rearing techniques; Paul Henry Mussen, John Janeway Conger, Jerome Kagan, and Aletha Carol Huston, Child Development and Personality, 7th ed. (1990); Marc H. Bornstein and William Kessen, Psychological Development from Infancy: Image to Intention (1979); Margaret B. Spencer, Geraldine Kearse Brookins, and Walter Recharde Allen (eds.), Beginnings: The Social and Affective Development of Black Children (1985), a collection of recent psychological studies; John Bowlby, Attachment and Loss, vol. 1, Attachment (1969, reissued 1982), a classic work, summarizing his theoretical ideas on the subject; Alexander Thomas and Stella Chess, Temperament and Development (1977), summarizing the research of two psychiatrists who reintroduced the concept of temperament; William Damon (ed.), Social and Personality Development: Essays on the Growth of the Child (1983), a comprehensive account of the social, familial, and cognitive determinants of a wide range of personal characteristics; Judy Dunn, Sisters and Brothers (1985), a survey of the impact of siblings on a child, and The Beginnings of Social Understanding (1988), presenting information based on observations of the home during the second year; John H. Flavell, Patricia H. Miller, and Scott A. Miller, Cognitive Development, 3rd ed. (1993), an excellent text; and Jean Berko Gleason (ed.), The Development of Language, 3rd ed. (1993), an excellent summary.The series titled The Handbooks of Aging comprises 3 vol., all 3rd ed. (1990): Edward L. Schneider and John W. Rowe (eds.), Handbook of the Biology of Aging; James E. Birren and K. Warner Schaie (eds.), Handbook of the Psychology of Aging; and Robert H. Binstock and Linda K. George (eds.), Handbook of Aging and the Social Sciences.Journals include Human Development (bimonthly), published in Switzerland; Developmental Psychology (bimonthly); Developmental Review (quarterly); Child Development (bimonthly); Journal of Experimental Child Psychology (monthly); and The International Journal of Aging & Human Development (8/yr.). Marc H. Bornstein Richard M. Lerner Jerome Kagan Development in adolescence Adolescence may be defined as that period within the life span when most of a person's characteristics are changing from what is typically considered childlike to what is typically considered adultlike. Changes in the body are the most readily observed, but other, less definitive attributes such as thoughts, behaviour, and social relations also change radically during this period. The rate of such changes varies with the individual as well as with the particular characteristic. Physiological aspects The physical and physiological changes of adolescence do not proceed uniformly; however, a general sequence for these changes applies to most people. It is useful to speak of phases of bodily changes in adolescence in order to draw important distinctions among various degrees and types of change. Bodily changes affect height, weight, fat and muscle distribution, glandular secretions, and sexual characteristics. When some of these changes have begun, but most are yet to occur, the person is said to be in the prepubescent phase. When most of those bodily changes that will eventually take place have been initiated, the person is in the pubescent phase. Finally, when most of those bodily changes have already occurred, the person is in the postpubescent phase; this period ends when all bodily changes associated with adolescence are completed. The bodily changes of adolescence relate to both primary and secondary sexual characteristics. Primary sexual characteristics are present at birth and comprise the external and internal genitalia (e.g., the penis and testes in males and the vagina and ovaries in females). Secondary sexual characteristics are those that emerge during the prepubescent through postpubescent phases (e.g., breasts in females and pigmented facial hair in males). Several important bodily changes occur specifically within each of the three periods that characterize adolescent physical maturation. The period of prepubescence begins with the first indication of sexual maturation. It ends with the initial appearance of pubic hair. In males, there is a continuing enlargement of the testicles, an enlargement and reddening of the scrotal sac, and an increase in the length and circumference of the penis. These changes all involve primary sexual characteristics. Insofar as secondary sexual characteristics are concerned, there is no true pubic hair at this stage, although down may be present. In females, prepubescent changes typically begin an average of two years earlier than in males. The first phenomena of female development in this period are the enlargement of the ovaries and the ripening of the ova. In contrast with those of males, these changes in primary sexual characteristics are not outwardly observable. However, changes involving secondary sex characteristics can be seen (e.g., the rounding of the hips and the first phase of breast development). The latter begins with an elevation of the areola surrounding the nipple, which produces a small conelike growth called the breast bud. As with the male, there is no true pubic hair, although down may be present. The onset of pubescence in both sexes occurs with the appearance of pubic hair, and this period ends when pubic hair development is complete. The peak velocity of growth in height and weight also occurs during this phase. This so-called growth spurt occurs about two years earlier in females than in males. Another key change of pubescence in females is menarche, or the onset of menstruation, which occurs about 18 months after the maximum height increase of the growth spurt and typically is not accompanied initially by ovulation. In pubescence the primary sexual characteristics continue the development initiated in prepubescence. In females the vulva and clitoris enlarge; in males the testes continue to enlarge, the scrotum grows and becomes pigmented, and the penis becomes longer and increases in circumference. In regard to secondary sexual characteristics, in females there is increased breast development, with the breast buds enlarging to form the primary breast; in males, the voice deepens and pigmented axillary and facial hair appear, usually about two years after the emergence of pubic hair. The phase of postpubescence starts when pubic hair growth is complete, a deceleration of growth in height occurs, changes in the primary and secondary sexual characteristics are essentially complete, and the person is fertile. Some changes in primary and secondary sexual characteristics occur in this phase. For instance, in males, it is during this period that the beard begins to grow; in females, there may be further breast development. Although, as noted, the ordering of these bodily changes is fairly uniform among individuals, there is considerable variation in the rate of change. Some adolescents mature more rapidly and others more slowly than most of their peers. Of course, there are also youths who pass through the periods of bodily change at the average rate. Variations in the rate of bodily change in adolescence often affect psychological and social development. Early-maturing adolescent boys are typically better adjusted than late maturers and have more favourable interactions with peers and adults. These advantages of early maturation and disadvantages of late maturation tend to continue through the middle adult years for males. For females, however, early maturation is associated with more psychosocial disadvantages than is late maturation. Maturing at an average rate seems to be most advantageous for females. However, the relations between female maturation rates and personality and social functioning in later life have not been determined. Bodily changes among adolescents can also differ according to sociocultural and historical influences. The age of menarche, for example, varies among countries and even among different cultures within one country. Moreover, there has been a historical trend downward in the average age of menarche, translating into a decrease of several months per decade from about 1840 to the present. This phenomenon is generally ascribed to the improved health and nutrition of children and adolescents. Development in adulthood and old age In sheer number of years, the periods labeled adulthood and aging constitute the major portion of the human life span. Historically, however, these periods were seen as less significant and interesting developmentally than infancy, childhood, and adolescence. Adulthood was viewed as a time of continuity, a period when what had been developed earlier was utilized. Aging was viewed as a time of decline, a period when what had been developed earlier was lost. Contemporary opinion is that adulthood and aging are just as significant and interesting as the earlier periods of the life cycle. Adulthood and aging are characterized by both growth and decline. Central nervous system processing There is relatively clear evidence that, with advancing age, individuals show a tendency toward decreasing speed of response. This is a gradual change occurring across the entire life span that shows up in a variety of so-called speeded tasks (those in which errors would be unlikely if the individual had an unlimited amount of time to complete the tasks). For example, reaction time tests (which measure the time elapsing between the appearance of a signal and the beginning of a responding movement) are usually viewed as a measure of central nervous system processing. Mean speed of response on such tasks increases with age until the late teens, remains constant until the mid-20s, and then declines steadily throughout the remainder of the age range. Much evidence has been accumulated to link changes in brain electrical activity to the slowing of behaviour. The electroencephalogram (EEG) provides a record of the brain's electrical activity. The normal human EEG displays continuous rhythmic activity in the form of wavelike patterns varying in frequency and amplitude. The dominant rhythm is the alpha wave, which reaches its maximum frequency in adolescence and begins to slow gradually after young adulthood. This slowing may be related to disease processes (particularly vascular disease) and to basic aging processes. The older adult's central nervous system appears to be in a state of underarousal in comparison to that of the younger adult. Development in childhood Language The capacity for language usually emerges in infants soon after the first birthday, and they make enormous progress in this area during their second year. Language is a symbolic form of communication that involves, on the one hand, the comprehension of words and sentences and, on the other, the expression of feelings, thoughts, and ideas. The basic units of language are phonemes, morphemes, and words. Phonemes are the basic sounds that are combined to make words; most languages have about 30 phonemes, which correspond roughly to the sounds of the spoken letters of the alphabet. Although one-month-old infants can discriminate among various phonemes, they are themselves unable to produce them. By 4 to 6 months of age, however, infants usually express vowellike elements in their vocalizations, and by 1112 months of age they are producing clear consonant-vowel utterances like dada and mama. Virtually all children begin to comprehend some words several months before they speak their own first meaningful words. In fact, one- to three-year-olds typically understand five times as many words as they actually use in everyday speech. The average infant speaks his first words by 1214 months; these are generally simple labels for persons, objects, or actions; e.g., mommy, milk, go, yes, no, and dog. By the time the child reaches his 18th month, he has a speaking vocabulary of about 50 words. The single words he uses may stand for entire sentences. Thus, the word eat may signify Can I eat now? and shoe may mean Take off my shoe. The child soon begins to use two-word combinations for making simple requests or for describing the environment: Want juice, Daddy gone, Mommy soup. These simple statements are abbreviated versions of adult sentences. Where is the ball? becomes where ball?; the sentence That's the ball becomes that ball. These early two-word combinations consist mostly of nouns, verbs, and a few adjectives. Articles (a, an, the), conjunctions (and, or, but), and prepositions (in, on, under) are almost completely absent at this stage. In their telegraphic sentences, children usually place the subject, object, and verb in an order that is correct within certain broad limits for their native language. For example, an American child will say want ball rather than ball want for a sentence meaning I want the ball. In the few months before the child's second birthday, there is a major increase in the size of his vocabulary and in the variety of his two- and three-word combinations. By two years of age a child's comprehension vocabulary contains an average of about 270 words. By the end of the second year, he understands interrogatives such as where, who, and what, and by three years of age he can correctly interpret the respective use of the words this or that and here or there, as well as the terms in front of and behind. By three years of age children are learning at least two new words a day and possess a working vocabulary of 1,000 words. Children in their second and third years sometimes use words as overextensions; doggie, for instance, may refer to a variety of four-legged animals as well as to dogs, and the word daddy may be used in reference to all men. This occurs simply because, although the infant detects the differences among various types of animals, he has only one word (dog) in his vocabulary to apply to them. Overextensions are more common in speech than in comprehension, however; the child who uses the word apple for all round objects has no difficulty pointing to an apple in a picture illustrating several round objects. Other words are underextended; that is, they are defined too narrowly. Some infants will use the word car to refer only to cars moving on the street but not to cars standing still or to a picture of a car. Children learn the rules of syntax (i.e., the grammatical rules specifying how words are combined in a sentence) with very little explicit instruction or tutoring from adults. They begin to flesh out their noun-verb sentences with less critical words such as prepositions, conjunctions, articles, and auxiliary verbs. Children follow a typical sequence in their acquisition of grammatical rules, depending on the language they are learning to use. In English, a child first masters the grammatical rules for the present tense (e.g., I want) and begins to use the present progressive ending (-ing) and the plural. This is followed by mastery of the irregular past tense (I made, I had), possessives (my, mine, his), articles (a, an, the), and the regular past tense (I walked, he stopped). These successes are followed by mastery of the third person present tense (he goes) and auxiliary verbs (I'm walking, we're playing). Deaf children learning sign language from deaf-mute parents show in their signs the same course of development that is apparent in the speech of children with normal hearing. Deaf, like hearing, children make their first signs for objects and later display signs for more complex ideas like Mommy eat or Daddy coat. By the middle of the third year, children tend to use more sentences containing four, five, or six words, and by the fourth year they can converse in adultlike sentences. Finally, five- and six-year-olds demonstrate metalinguistic awarenessi.e., a mastery of the complex rules of grammar and meaning. They can differentiate between sounds that are real words and those that are note.g., they regard apple as a word but reject oope as a word. They can tell the difference between grammatically correct and incorrect sentences and will make spontaneous corrections in their speech; that is to say, if a child makes a speech error, he recognizes it and will say the phrase or sentence correctly the second time. A major disagreement among theories of language acquisition is their relative emphasis on the role of maturation of the brain, on the one hand, and of social interaction, on the other. The most popular view assumes that biological factors provide a strong foundation for language acquisition but that infants' social interaction with others is absolutely necessary if language is to develop. The special biological basis of language is supported by the fact that deaf children who are not exposed to a sign language invent a symbol system that is similar in structure to that developed by hearing children. But interaction with other people is also crucial. Even during the first year, children's production and perception of speech sounds are increasingly shaped by the linguistic environment around them, reflecting their exquisite sensitivity and susceptibility to human speech. Indeed, the amount and variety of verbal stimulation is a critical factor in language development, as is the adult caregivers' sensitivity to an infant's own vocalizations; mothers who ask questions and encourage their infants' vocal responses have children who show a more advanced language development. Cognitive development The mental activities involved in the acquisition, processing, organization, and use of knowledge are collectively termed cognition. These activities include selective attention, perception, discrimination, interpretation, classification, recall and recognition memory, evaluation, inference, and deduction. The cognitive structures that are involved in these processes include schemata, images, symbols, concepts or categories, and propositions. A schema is an abstract representation of the distinctive characteristics of an event. These representations are not photographic copies or visual images but are more like schematic blueprints that emphasize the arrangement of a set of salient elements, which supply the schema with distinctiveness and differentiate it from similar events. The child's ability to recognize the face of another person is mediated by a schema, for example. Young children already display a remarkable ability to generate and store schemata. Another type of early cognitive unit is the image; this is a mental picture, or the reconstruction of a schema, that preserves the spatial and temporal detail of the event. Symbols represent the next level of abstraction from experience; they are arbitrary names for things and qualities. Common examples of symbols are the names for objects, letters, and numbers. Whereas a schema or image represents a specific experience, such as a sight or sound, a symbol is an arbitrary representation of an event. The letter A is a symbol, and children use schemata, images, and symbols in their mastery of the alphabet. Symbols are used in the development of higher cognitive units called concepts. A concept, or category, may be thought of as a special kind of symbol that represents a set of attributes common to a group of symbols or images. The concept represents a common attribute or meaning from a diverse array of experiences, while a symbol stands for a particular class of events. Concepts are used to sort specific experiences into general rules or classes, and conceptual thinking refers to a person's subjective manipulations of those abstract classes. Jean Piaget tried to trace specific stages in children's progressive use of symbols and concepts to manipulate their environment. According to Piaget, two of the four stages of cognitive development occur during childhood: the preoperational stage (2 to 7 years), in which the child learns to manipulate the environment by means of symbolic thought and language; and the concrete-operational stage (7 to 12 years), in which the beginnings of logic appear in the form of classifications of ideas and an understanding of time and number. An important structure in Piaget's theory of cognitive development is the operation, which is a cognitive structure that the child uses to transform, or operate on, information. Children learn to use operations that are flexible and fully reversible in thought; the ability to plan a series of moves in a game of checkers and then mentally retrace one's steps to the beginning of that sequence is one such example of an operation. It is important to make a distinction between the knowledge and skills a child possesses, called competence, and the demonstration of that knowledge in actual problem-solving situations, called performance. Children often possess knowledge that they do not use even when the occasion calls for it. Adapting to new challenges, according to Piaget, requires two complementary processes. The first, assimilation, is the relating of a new event or object to cognitive structures the child already possesses. A five-year-old who has a concept of a bird as a living thing with a beak and wings that flies will try to assimilate the initial perception of an ostrich to his concept of bird. Accommodation, the second process, occurs when the information presented does not fit the existing concept. Thus, once the child learns that the ostrich does not fly, he will accommodate to that fact and modify his concept of bird to include the fact that some birds do not fly. One of the central victories of cognitive development occurs during ages five to seven and, according to Piaget, marks the child's entry to the concrete-operational stage. This is the ability to reason simultaneously about the whole and about part of the whole. For instance, if an eight-year-old is shown eight yellow candies and four brown candies and asked, Are there more yellow candies or more candies, he will say that there are more candies, whereas a five-year-old is likely to respond incorrectly that there are more yellow candies. A child who has reached the concrete-operational stage is able to solve several other new kinds of logical problems. For example, a five-year-old who is shown two balls of clay of the same size and shape will tell an adult that they have the same amount of clay, but, when the experimenter rolls one of the balls into a long but thin sausage, the five-year-old will tend to say that the untouched sphere has more clay in it than the sausage-shaped object does. A seven-year-old, however, shows what is called the ability to conserve; when presented with the same problem, he will recognize that the two pieces still have the same amount of clay in them, based on his awareness that liquids and solids do not change in amount or quantity merely because their external shape changes. The seven-year-old is able to reverse an event in thought and knows that the sausage can be reshaped back into the original ball without a loss or gain in the total amount of clay. The knowledge that one can reverse one state of affairs into a prior state, which is called conservation, is a mark of this new stage of development. Another cognitive advance children make during the concrete-operational stage is the knowledge that hierarchical relationships can exist within categories. This is illustrated by the ability to arrange similar objects according to some quantified dimension, such as weight or size. This ability is called seriation. A seven-year-old can arrange eight sticks of different lengths in order from shortest to longest, indicating that the child appreciates a relation among the different sizes of the objects. Seriation is crucial to understanding the relations between numbers and hence to learning arithmetic. Children in the concrete-operational stage also appreciate the fact that terms such as taller, darker, and bigger refer to a relation between objects rather than to some absolute characteristic. One implication of the stage of concrete operations is that the child is now able to compare himself with other children in such qualities as size, attractiveness, intelligence, courage, and so on. Hence, the formation of the child's sense of identity, or self-concept, proceeds at a faster rate because he is able to compare his characteristics with those of other children. The final stage of cognitive development, called the stage of formal operations, begins at about age 12 and characterizes the logical processes of adolescents and adults. A child who has reached this stage of logical thinking can reason about hypothetical events that are not necessarily in accord with his experience. He shows a willingness to think about possibilities, and he can analyze and evaluate events from a number of different possible perspectives. A second hallmark of the stage of formal operations is the systematic search for solutions. Faced with a novel problem, the adolescent is able to generate a number of possible means of solving it and then select the most logical, probable, or successful of his hypotheses. The formal thinking of adolescents and adults thus tends to be self-consciously deductive, rational, and systematic. Finally, adolescents typically begin to examine their own thinking and evaluate it while searching for inconsistencies and fallacies in their own beliefs and values concerning themselves, society, and nature. Development in infancy 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. The newborn infant 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 infant's emotional responses to other people.