The sexual development of human beings involves certain basic biological processes common to the human race. Such processes begin with conception and continue on into maturity. Social influences are also involved in sexual development, however. Such influences may vary from culture to culture according to value systems, stereotypes of masculinity and femininity, conventions and taboos of sexual behavior, and child-rearing philosophies shared and taught by different cultural groups. Psychological attitudes and emotions also affect sexual development, and these can vary from individual to individual. Cultures differ in the subtle differences in sexual attitudes conveyed to children by their families, and no two families are ever quite identical. Emotions arise in response to family and other social experiences. These emotions can influence sexual development powerfully, because when they arise they color all subsequent responses to later experiences. Because biological, social, and psychological influences all continue to change throughout the life of a human being, sexual development is a process that never ends. PRENATAL SEXUAL DEVELOPMENT Sexual \Tdevelopment\t begins at conception. The female ovum contributes an X chromosome to the embryo. If the male sperm contributes another X chromosome, the embryo will develop ovaries and become a female. If the sperm instead contributes a Y chromosome, the embryo will begin to develop androgen-producing testes at 6 weeks and become a male. (Androgen is a male \Tsex hormone\t.) Geneticists have found strong circumstantial evidence that a single gene on the Y chromosome, called the testes determining factor (TDF), determines this outcome. Thus an abnormal Y chromosome that lacks TDF can result in a fetus with XY chromosomes that even so develops as a female. Alternatively, an abnormal X chromosome that contains TDF can result in a fetus with chromosomes that nevertheless develops as a male. Under the influence of androgens, male sex organs are formed by 14 weeks. Additionally, the androgens organize parts of the developing male brain so that the individual will exhibit behavior that is statistically more typical of males than females; after puberty the male sex hormones will be produced steadily, and innately the male will be more easily aroused sexually by a female than by a male. In female fetuses no androgens are produced, and this permits female genitals and internal organs to form during the same early period. Androgen absence also leads to organization of parts of the developing female brain so that behavior patterns predominate that are statistically more typical of females than males. Female sex hormones will be produced in a cyclic pattern (which accounts for the menstrual cycle), and a similar bias toward heterosexual arousal will exist. Behavioral scientists disagree about the nature and extent of innate sex-specific behavior and attitudes. Pervasive cross-cultural similarities and other human and animal research suggest that some sex-specific characteristics are innate but that they are not mutually exclusive. All behaviors and attitudes can normally exist in either sex, but some patterns are statistically more common in one or the other. INFANCY Newborn infants of either sex are much more alike than different but do show some sex-specific differences. Statistically boys are larger at birth, can raise their heads higher than girls, are more irritable and difficult to calm, and within a few months show more anger and aggression. Older male infants are less fearful and more active in strange situations and cling less to their mothers. These attributes correlate with the greater physical aggressiveness and use of energy in males of all ages cross-culturally and also in primates and other mammals. Girls are quieter and more sensitive to touch, smile earlier and more often (even in sleep), are more responsive to social stimuli, and talk more and earlier than boys. They are less exploratory and more fearful in strange situations. It is difficult to separate innate differences from learned responses to the different ways parents act toward boys and girls beginning at birth. Mothers are more attentive to the fussiness of boys and play more vigorously with them, whereas they elicit more vocalizing and smiling from girls. With older infants mothers tolerate more aggression and independence in boys. These same differences occur in widely divergent cultures and other species (rhesus monkey mothers push their male infants away early while still keeping females close), suggesting that parental behavior does not spring simply from preconceptions about masculinity or femininity but responds to and fosters innate sex-specific temperaments and behaviors. From birth, parents and society begin to teach an infant that it is male or female and, as it grows older, what the different expectations are of each sex. By 2 years of age, children normally have an unchangeable inner sense of being male or female. This early learning is also the beginning of the wide cultural and individual variations in the accepted norms of maleness and femaleness. Whatever sex-specific qualities are innate are extremely fragile and are easily modifiable and can even be distorted into sexual deviations by postnatal learning influences. EARLY CHILDHOOD With good, physically affectionate parenting, infants learn that their own and others' bodies are good and that physical closeness may be trusted. Next, during the separation and individuation of toddlerhood, children may learn that they own their bodies, which includes the right to all body parts and sensations. Toddlers discover genital sexual differences and the pleasure of touching their genitals. The attitudes of parents toward a child's sexual curiosities and genital fondling determine whether the child accepts his or her genitals and sensations as good and prideful or bad and shameful. From this time on, each sex must learn what it means, personally and socially, to be male or female. The chiefly internal and invisible female genitalia and erotic sensations, or the external and visible male organs with their readily localized sensations, play important roles in how each sex conceptualizes itself and in the different sexual capacities and vulnerabilities each sex feels. Initially, all infants identify with the mother, but during toddlerhood they begin to identify appropriately with the same-sex parent. In modern industrial societies this task is harder for boys, whose fathers usually work away from home and who are absent as male models during most of a boy's waking hours. It is also normal for toddlers to express wishes to be like the opposite-sex parent, although persistent cross-sex identification is abnormal. Toddlers begin to be socialized to conform with the family and societal norms for their sex, and their self-concepts are influenced by what they sense of how one or the other sex is valued at home. In most respects the sexes are more alike than different, and early sex role stereotyping may be harmfully constricting when it inculcates mutually exclusive differences, such as avoidance of tenderness in boys or avoidance of assertiveness and sports in girls. Between 3 and 5 years of age, emotional development and neurobiological maturation cause genital erotic sensations to become much more intense. Masturbation becomes considerably more frequent, especially in boys, although the difference is less marked when girls are not subject to more stringent sexual prohibitions. The child becomes jealous of the parents' sexual intimacy and quite naturally wants to satisfy the new sexual urges with the most-loved opposite-sex person: mother or father (the Oedipus complex). Much of this is unconscious or expressed only in dreams, but most parents can recall such comments as, "When I grow up and Daddy dies, I'm going to marry Mommy," or, "I want to have a baby with Daddy, too!" At the same time, children feel guilty about their wishes to get rid of the same-sex parent whom they also love, and they also fantasize that this bigger and more powerful rival will retaliate by destroying the child's sexuality. The healthy outcome of this inner conflict is that the child accepts the impossibility of the wish, relinquishes the desire to displace the same-sex parent, and instead firmly and definitively identifies with mother or father. Obscure as this conflict is, the support of parents' realistic attitudes as a corrective to the child's exaggerated fantasies is crucial to healthy sexual development. Unfortunately, some parents become uneasy over their child's often blatant sexuality and either withdraw or sharply disapprove of it; to the child, this kind of reaction confirms that the newly experienced heterosexual wishes are bad and dangerous. If the parents are welcoming toward the child's budding sexuality, however, and accepting of the normalcy of erotic feelings and masturbation while gently indicating that the desires must be deferred until later and to a more appropriate future partner, the child will achieve a major component of sexual identity. The child will have learned that he or she has the right to heterosexual desires, to eventual competition for a partner, and to sexual fulfillment. It is probable that future heterosexual or homosexual object choice is determined during this stage. LATE CHILDHOOD Although begun earlier, the most intense cultural socializing toward accepted sex-role norms is during the early school years. The child is no longer exposed only to possibly idiosyncratic family attitudes and now confronts the broader community's expectations of future men and women. In their identification with parents and adults of the same sex, children learn how males and females are supposed to act and feel, how they regard one another, and what the accepted ranges of interests and future occupations are for each sex; children are discouraged, punished, or ridiculed for deviations from the local norm. Wide divergence in the details of masculine and feminine norms occurs in different cultures but most cultures intuitively socialize children to sex roles that are consonant with innate propensities and natural complementary life functions. When cultural norms exaggerate the benign innate complementary sex roles into cripplingly restrictive stereotypes, the youngster may be condemned to a limited expression of potential or may reject his appropriate sex role totally, usually with serious emotional and social consequences. It is also well to realize that for more than 99% of hominid evolution, humans lived in a hunting and gathering culture. Whatever masculine and feminine traits are innate are largely heritages from that kind of culture and may or may not have the same adaptive value in contemporary society. PUBERTY Puberty is the surge of biological maturation that culminates in reproductive capacity and relatively adult body size and contour; adolescence is the psychosocial response to puberty. Puberty occurs about 2 years earlier for girls than for boys, and the full span of bodily changes may require from 1-1/2 to 4 years. Usually in girls at about 10 or 11 years of age the sex hormone estrogen is produced in an increasing amount by the maturing ovaries, initiating breast and uterine development and the typical distribution of fat that results in characteristic feminine body contours. Females also produce the male hormone androgen, which is responsible for the development of pubic and axillary hair and the enlargement of the clitoris and labia majora. Androgen is also responsible for the erotic desire and intensity of genital sensation in women. At about the midpoint of this process (average 12 - 12-1/2 years), the hypothalamus has begun its cyclic regulation of sex hormones and the first \Tmenstruation\t occurs. In boys the testicles begin to enlarge at about 12 years and about a year later begin to produce increasing amounts of testosterone. This production initiates growth of the penis, pubic hair, and prostate, and deepening of the voice, and it is responsible for the characteristic male musculature and heavier, broader bone growth. The ability to ejaculate semen with viable sperm--the male equivalent of the first menstruation--usually comes slightly before 14 years of age. Puberty may occur as much as 2-1/2 years earlier or later than the average and still be normal. Markedly early or late puberty, however, may cause great emotional distress to the person who is so out-of-step with peers. ADOLESCENCE The change from a child to a person with an adult body and sex drives places stress on most individuals, in Western society at least, and the years of adolescence are required for integration. Girls are often readier to relate socially to older boys because of their male peers' lag in maturation. Boys tend to band together and avoid girls in early adolescence. The Oedipal attraction to opposite-sex parents reemerges and contributes to both sexes going through a phase of intense emotional distancing from parents. The surge of sex hormones tends at first to intensify male-female differences, and even most ex-tomboys become more traditionally feminine as that surge is reinforced by male peers. Society, too, aware of the reproductive and work potential of the adolescent, increases its pressure for conformity to its preferred norms of masculinity and femininity. This demand may cause both social and emotional hardships in periods of cultural transition for those who want to extend the bounds of sex role stereotypes. In the West, prohibitions of normal childhood sexuality and inadequate sex education ill prepare children for the stronger sexual feelings of puberty. The need for sexual exploration, though, is powerful, and masturbation increases, again more in boys than girls, probably because of continuing double standards. Masturbation is healthy and beneficial in providing means for learning about an individual's body and sexual responses and correlates positively with later sexual response with a partner. The earliest sexual activity with others is more often with friends of the same sex, because they are more familiar and less anxiety producing; this kind of homosexual activity seldom indicates future homosexuality but may worry the participants and parents. As they become more secure, youngsters begin heterosocial activities, such as group parties and dates, along with some heterosexual experimentation. By the mid- to late teens, pair dating has become the preferred heterosocial activity for most. The trend recently has been for adolescents to begin sexual activity earlier. Once youngsters begin petting and genital fondling, they will usually experiment with intercourse within a few months, although some do not and remain virgin until adulthood or marriage. Although masturbation remains the chief mode of sexual release throughout adolescence, even for those who are heterosexually active, a few begin intercourse by 12 or 13 years. The proportion with intercourse experience varies in different geographical and subcultural groups, hence the disagreement between different surveys. In most areas by the end of high school, half or more adolescents have had intercourse, and--a significant shift from the past--the proportion is often higher among girls. By the end of the teens young people who have had intercourse at least once are in a clear majority. In early adolescence the newly intense sex drive and a person's own body and responses are so unfamiliar that sexual activity is mainly self-centered, even when with a partner. The adolescent may believe he or she is devoted, but the real developmental need is to explore one's own sexual functions, sensations, and responses. With experience these become familiar and secure, and one turning point into late adolescence is the emotional capacity to shift focus to the partner, the relationship, and the meaning of emotional involvement. Adolescents, particularly young adolescents, still retain some nonlogical thought habits from childhood, and this retention is one reason for the frequent failure to think of real consequences to oneself or another, such as pregnancy or venereal disease. Another reason for a disregard of consequences is the guilt most still feel, despite protestations to the contrary, so that they cannot consciously and rationally plan for sex responsibly. Sexual experience in adolescence, however, is natural and not always emotionally or practically damaging, although not necessary for sexual health. Experience is important to any learning. For those adolescents who are capable of realistic thinking and can accept their sexuality with minimum conflict, it is rewarding and nondestructive. ADULTHOOD Sexual development does not end with adolescence but continues through life. Young adults expand their emotional development to cope with extended commitment to another person and balance their sexual drives with their sexual values. Marriage adds the new dimension of trying to synchronize two persons' sexual development to maximize the positive bond and minimize disruption. Children trigger a whole new cycle of sexual development. Parenthood permits a new dimension of identification with a person's own parents and perhaps a resolution of residual conflict that has impaired optimal maturity. As children go through each sexual development phase, it reawakens unfinished emotional business from that phase in the parent, permitting further growth that benefits both generations. In adult sexual interactions it is probably not a biological truth that male sex drive peaks in the late teens and then gradually declines, and female sex drive gradually rises into the forties. More probably that concept has resulted from the traditionally more stringent prohibitions against girls learning to be sexual. In their late forties and early fifties most women cease menstruating and can no longer bear children. For a minority this time is difficult; women in this group feel their usefulness is over, especially if having children was their only source of self-approbation. Most women, however, take it in stride or even discover a new freedom. There is no such sudden decline in male fertility, men having fathered children when in their nineties. Men may also experience emotional crisis, however, if they feel in danger of losing vigor and sexuality. In our culture there is a myth that older people are not, and should not be, sexual. This notion is untrue; only the imperative nature of the drive gradually relaxes. With an available compatible partner and in the absence of severe physical disorder, full sexual gratification is a fulfilling interaction to the end of life. Warren J. Gadpaille Bibliography: Beach, F. A., ed., Human Sexuality in Four Perspectives (1976); Friedman, R. C., et al., eds., Sex Differences in Behavior (1974); Gadpaille, Warren J., The Cycles of Sex (1975); Hutchison, J. B., ed., Biological Determinants of Sexual Behavior (1978); Maccoby, E. E., and Jacklin, C. M., The Psychology of Sex Differences (1974); Money, John, and Ehrhardt, Anke, Man and Woman, Boy and Girl: Differentiation and Dimorphism of Gender Identity from Conception to Maturity (1973). See also: \Tsex\t; \Tsex education\t; \Tsex hormones\t.