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The full range of thoughts and actions that describe sexual motivation and behavior.

While sex is not necessary for an individual's survival, without it a species would cease to exist. The determinants of sexual motivation and behavior include an individual's physiology, learned behavior, the physical environment, and the social environment.

A person's sex is determined at conception by whether one out of the 23 chromosomes in the father's sperm is either X (female) or Y (male). All female eggs contain an X chromosome, so each fertilized egg, or embryo, has a genotype of either XX (female) or XY (male). Reproductive hormones produced by the gonads (male testes and female ovaries) determine the development of the reproductive organs and the fetal brain, especially the hypothalamus. All the human reproductive hormones are found in both sexes but in different amounts. The principal female hormones are estrogens and progesterone (of which the main ones are estradiol and progesterone); the primarily male hormones are androgens (mainly testosterone). In males, levels of testosterone remain fairly constant, regulated by a feedback loop to the brain and pituitary gland, which control hormone secretion. In females, hormone levels fluctuate within each menstrual cycle, rising at ovulation. Reproductive hormones have two types of effects on the body. Organizational effects, which occur primarily before birth, are irreversible and permanently govern an individual's response to further hormone secretion. Activational effects govern behavior temporarily while hormone levels are elevated.

Human females are born with about 400,000 immature eggs. Each one is contained in a sac called a follicle. When a girl reaches puberty, one or more eggs mature every month, stimulated by the release of a hormone from the pituitary gland. As the egg matures, it secretes the hormone estrogen, causing the uterine lining to thicken in anticipation of the implantation of a fertilized egg. This is followed by ovulation, as the follicle ruptures, releasing the mature egg which travels through the fallopian tube towards the uterus. If the egg is not fertilized by sperm, it disintegrates and the uterine lining leaves the body, a process called menstruation. Women remain fertile until menopause, which normally occurs around the age of fifty. Unlike the production of female eggs, the male production of sperm is not cyclical and men remain fertile throughout their lives, although they may produce fewer sperm as they age. A man produces several billion sperm each year, releasing 300 to 500 million sperm in an average ejaculation.

Unlike that of other species, human sexual behavior is not bound to the female reproductive cycle. Women may engage in or refrain from sexual intercourse at any time during the cycle. Some women have reported increased sexual interest at the time of ovulation, others around the time of menstruation, and still others experience no link at all between their sexual behavior and menstrual cycle. After their initial organizational effects at birth, hormone levels stay low until puberty when activational effects first begin, triggering the reproductive system and generating an interest in sexual behavior. Whether or not sexual activity actually occurs at this point, however, depends on the interaction of physical readiness, social skills, and opportunity. For adults, as for adolescents, sexuality is not governed solely by hormones but also by a repertoire of learned attitudes and behaviors. This learning begins in childhood with the development of gender roles and continues throughout the life span, and it depends on attitudes prevalent in a culture at a given time.

The laboratory research conducted by William Masters and Virginia Johnson in the 1950s and 1960s yielded important information about the human cycle of sexual response. This cycle has four phases for both men and women: initial excitement, a plateau stage, orgasm, and resolution, during which the person returns to a state of relaxation. Males experience a refractory period after orgasm during which they are temporarily insensitive to sexual stimulation.

The same combination of physical, psychological, and social factors that govern sexuality may contribute to sexual dysfunction, any condition that inhibits the desire for, or ability to have, satisfying sexual experiences. In males, the most common dysfunction is impotence, or the inability to have or maintain an erection sufficient for intercourse. While impotence can have physical origins, including fatigue, diabetes, alcoholism, and the side effects of certain medications, it is usually psychological in nature. In females, a common sexual dysfunction is the inability to reach orgasm, also called arousal disorder, which is also associated with such psychological factors as self-consciousness, lack of self-confidence, depression, and dissatisfaction with the nature of the romantic relationship itself.

Although human sexual activity is primarily heterosexual, between 5 and 10 percent of males and 2 to 6 percent of females in the United States are homosexuals, individuals in whom sexual attraction and behavior are directed at members of their own sex. (Persons whose sexual behavior is directed at members of both sexes are known as bisexuals.) Researchers have found evidence of both biological and environmental origins of homosexuality. While no significant differences have been found in the levels of hormones that circulate in the blood of homosexuals and heterosexuals, exposure to high levels of certain reproductive hormones during fetal development has been linked to homosexuality. In addition, anatomical differences have been found between the hypothalamus of heterosexual and homosexual men, and studies of twins have found distinct evidence of a hereditary component to homosexuality. Environmental influences include early family relationships and the modeling of behaviors observed in the parent of the opposite sex, as well as social learning throughout the life span.

Sexual preference—the gender to which one is attracted—is only one aspect of human sexual orientation. Also involved is gender role, a general pattern of masculine or feminine behaviors that is strongly influenced by cultural factors. Distinct from this is sex identity, referring to whether individuals consider themselves to be male or female. Transsexualism, a condition in which a person believes he or she is of the wrong sex, occurs in approximately one in 20,000 in men and one in 50,000 in women. Today, these individuals have the option of a sex change operation that allows them to live as a member of the sex with which they identify.

Further Reading

Fisher, Seymour. Sexual Images of the Self: The Psychology of Erotic Sensations and Illusions. Hillsdale, NJ: L. Erlbaum Associates, 1989.

Levand, Rhonda. Sexual Evolution. Berkeley, CA: Celestial Arts, 1991.

Additional topics

Psychology EncyclopediaPsychological Dictionary: Perception: early Greek theories to Zombie