Human Sexuality, general term referring
to various sexually related aspects of human life, including physical and
psychological development, and behaviors, attitudes, and social customs
associated with the individual's sense of gender, relationships, sexual
activity, mate selection, and reproduction. Sexuality permeates many areas of
human life and culture, thereby setting humans apart from other members of the
animal kingdom, in which the objective of sexuality is more often confined to
reproduction. This article discusses the sexual anatomy, development,
physiology, and behavior of human beings.
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HUMAN
SEXUAL CHARACTERISTICS
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Sexual characteristics
are divided into two types. Primary sexual characteristics are directly
related to reproduction and include the sex organs (genitalia). Secondary
sexual characteristics are attributes other than the sex organs that
generally distinguish one sex from the other but are not essential to
reproduction, such as the larger breasts characteristic of women and the facial
hair and deeper voices characteristic of men.
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Female
Sexual Organs
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Primary sexual characteristics
of women include the external genitalia (vulva) and the internal organs that
make it possible for a woman to produce ova (eggs) and become pregnant. The
vulva includes the mons pubis, the most visible part of the woman's
external genitalia, which is the pad of fatty tissue that covers the pubic bone
and is commonly covered by pubic hair; the labia majora, the large outer
lips; and the labia minora, the smaller, hairless inner lips that run
along the edge of the vaginal opening and often fold over to cover it. The
labia minora come together in front to form the clitoral hood, which covers the
clitoris, a sensitive organ that is very important to the woman's sexual
response. The opening of the urethra, the tubular vessel through which
urine passes, is located midway between the clitoris and the vaginal opening.
The area where the labia majora join behind the vagina is called the fourchette.
The area of skin between the vaginal opening and the anus is the perineum.
The hymen is a thin membrane that partially covers the vaginal opening.
If the hymen is extensive and is still present at first intercourse, it may be
broken or stretched as the penis enters the vagina and some bleeding and pain
may occur, although more typically its presence is unnoticed. The presence or
absence of a hymen is not a reliable indicator of virginity, although historically
it was viewed as such.
The internal sex organs
of the female consist of the vagina, uterus, fallopian tubes (or oviducts), and
ovaries. The vagina is a flexible tube-shaped organ that is the
passageway between the uterus and the opening in the vulva. Because during
birth the baby travels from the uterus through the vagina, the vagina is also
known as the birth canal. The woman's menstrual flow comes out of the uterus
and through the vagina. When a man and a woman engage in vaginal intercourse,
the penis is inserted into the vagina.
The cervix is located
at the bottom of the uterus and includes the opening between the vagina and the
uterus. The uterus is a muscular organ that has an inner lining
(endometrium) richly supplied with blood vessels and glands. During pregnancy,
the uterus holds and nourishes the developing fetus. Although the uterus is
normally about the size of a fist, during pregnancy it is capable of stretching
to accommodate a fully developed fetus, which is typically about 50 cm (about
20 in) long and weighs about 3.5 kg (about 7.5 lbs). The uterine muscles also
produce the strong contractions of labor.
At the top of the uterus
are the pair of fallopian tubes that lead to the ovaries. The two ovaries
produce eggs, or ova (the female sex cells that can become fertilized), and
female sex hormones, primarily estrogens and progesterone. The fallopian tubes
have fingerlike projections at the ends near the ovaries that sweep the egg
into the fallopian tube after it is released from the ovaries. If sperm are
present in the fallopian tube, fertilization (conception) may occur and the
fertilized egg will be swept into the uterus by cilia (hairlike
projections inside the fallopian tube).
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Male
Sexual Organs
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The external sex organs
of men are the penis and the scrotum. The penis is a sensitive organ
important to reproduction and urination and to sexual pleasure. At its tip is
the glans, which contains the urethral opening, through which
urine passes. The ridge that separates the glans from the body of the penis is
called the corona (Latin for “crown”), or coronal ridge. The
glans and the corona are the most sensitive parts of the penis. The glans is
covered with a foreskin (prepuce) unless the man has been circumcised,
in which case the foreskin has been surgically removed.
The penis contains three
cylinders of tissue that run parallel to the urethra. During sexual arousal,
these tissues become engorged with blood and expand, causing the penis to
enlarge and become erect (erection or tumescence). Men do not have a penis bone
or a muscle that causes erection, as do some other animals.
The scrotum is a pouch
that hangs below the penis and contains the two testes, which produce sperm
(the male sex cell responsible for fertilization) and are considered part of
the internal genitalia. The testes also are the primary producers of testosterone
(male sex hormone) in men. Inside the testes are about 1,000 seminiferous
tubules that manufacture and store the sperm. The scrotum can pull up
closer to the body when the surrounding temperature is low and can drop farther
away when the temperature is hot in order to keep the testes at an optimal,
constant temperature somewhat lower than body temperature.
After sperm are produced,
they move out of each testis and into the epididymis, a long tube coiled
against the testis, where the sperm are stored and mature. The vas deferens
transports the sperm from the epididymis through the prostate, after which the
vas deferens becomes the ejaculatory duct. Here, fluids from the prostate and seminal
vesicles (small sacs that hold semen) combine with the sperm to form semen,
a thick, yellowish-white fluid. The average discharge of semen, called
ejaculate, contains approximately 300 million sperm.
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SEXUAL
DEVELOPMENT
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There are two periods
of marked sexual differentiation in human life. The first occurs prenatally and
the second occurs at puberty. Although adult women and men may differ greatly
in genital appearance and secondary sexual characteristics, they are almost
identical during prenatal development. When an egg and a sperm unite during
fertilization, they each bring to the new cell half the number of chromosomes
(threadlike structures that contain genetic material) present in other cells.
From fertilization through about the first six weeks of development, male and
female embryos differ only in the pair of sex chromosomes they have in each
cell—two X chromosomes (XX) in females and one X and one Y chromosome (XY) in
males. At this stage, both male and female embryos have undifferentiated gonads
(ovaries or testes), two sets of ducts (one set capable of developing
into male internal organs and the other into female organs), and
undifferentiated external genital folds and swellings.
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Prenatal
Sexual Development
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About six weeks after
conception, if a Y chromosome is present in the embryo's cells (as it is in
normal males), a gene on the chromosome directs the undifferentiated gonads to
become testes. If the Y chromosome is not present (as in normal females), the
undifferentiated gonads will become ovaries.
If the gonads become testes,
they begin to produce androgens (male hormones, primarily testosterone)
by about eight weeks after conception. These androgens stimulate development of
the one set of the genital ducts into the epididymes, vas deferens, and ejaculatory
ducts. The presence of androgens also stimulates development of the penis and
the scrotum. The testes later descend into the scrotum. Males also produce a
substance that inhibits the development of the second set of ducts into female
organs. In the absence of such hormonal stimulation, female structures develop.
Prenatal hormones also
play a role in the sexual differentiation of the brain. For example, prenatal
hormones direct the development of sex differences in some cells and the neural
pathways in the hypothalamus (the part of the brain that controls the endocrine
system). Beginning at puberty, based on prenatal sexual differentiation, the
hypothalamus directs either the cyclic secretion of sex hormones that controls
the female menstrual cycle or the relatively continuous production of male sex
hormones. Other brain differences may be related to differences in sexual and
aggressive behavior or in cognitive and perceptual characteristics. Most of the
research on sexual differentiation of the brain has been performed with animals
or with biased human samples, and there is much debate about the nature and
behavioral relevance of these differences in humans.
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Childhood
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After birth, the process
of sex-role socialization begins immediately. There may be small,
physiologically-based differences present at birth that lead girls and boys to
perceive the world or behave in slightly different ways. There are also
well-documented differences in the ways that boys and girls are treated from
birth onward. The behavioral differences between the sexes, such as differences
in toy and play preference and in the degree of aggressive behavior, are most
likely the product of complex interactions between the way that the child
perceives the world and the ways that parents, siblings, and others react to
the child. The messages about appropriate behavior for girls and boys intensify
differences between the sexes as the child grows older.
It is not uncommon for
children to touch or play with their genitals or to play games, such as
“doctor” or “house,” that include sexual exploration. Such experiences are
usually not labeled sexual by the children. Adults will often discourage such
behavior and respond negatively to it. Generally by the age of six or seven,
children develop a sense of privacy and are aware of social restrictions on
sexual expression.
As the first bodily changes
of puberty begin, sometime from the age of 8 to the age of 12, the child may
become self-conscious and more private. During this period, more children gain
experience with masturbation (self-stimulation of genitals). Surveys indicate
that about one-third of all girls and about half of all boys have masturbated
to orgasm by the time they reach the age of 13, boys generally starting earlier
than girls. Because preadolescents tend to play with others of their own sex,
it is not at all uncommon that early sexual exploration and experience may
happen with other members of the same sex.
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Puberty
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Puberty marks the second
stage of physical sexual differentiation—the time when both primary and
secondary sexual characteristics as well as adult reproductive capacity
develop, and when sexual interest surges. Puberty typically begins in girls
from 8 to 12 years of age, whereas boys start about two years later. The
hypothalamus initiates pubertal changes by directing pituitary growth hormones
and gonadotropins (hormones that control the ovaries and testes).
A girl's breasts grow,
her pubic hair develops, and her body grows and takes on the rounded contours
of an adult woman. This is followed by the first menstrual period (menarche) at
about age 12 or 13 (although ages of onset range from 10 to 16.5),
underarm-hair growth, and increased secretions from oil- and sweat-producing
glands. It may take a year or two before menstruation and ovulation occur
regularly. The hormones primarily responsible for these changes in young girls
are the adrenal androgens, estrogens, progesterone, and growth hormone.
During puberty, a boy's
testes and scrotal sac grow, his pubic hair develops, his body grows and
develops, his penis grows, his voice deepens, facial and underarm hair appear,
and secretions from his oil- and sweat-producing glands increase. Penile erections
increase in frequency, and first ejaculation (thorarche) typically occurs
sometime from the age of 11 to the age of 15. For a boy who has not
masturbated, a nocturnal emission, or so-called wet dream, may be his first
ejaculation. The ability to produce sperm may take another year or two and
typically begins at about age 14. Growth hormone and androgens, particularly
testosterone, are responsible for these pubertal changes in boys.
The fact that boys tend
to develop more slowly than girls can cause some social awkwardness. Girls who
have grown earlier may find themselves much taller than their dates, for
example, and they may be more physically and psychologically mature than their
male peers.
The first menstruation
and first ejaculation are often considered the most important events of
puberty, particularly for the individual. However, it is the development of the
secondary sexual characteristics that serve as more apparent signals to others
that the person is becoming a man or a woman. These signals lead to increasingly
differential treatment of adolescent girls and boys by parents or other adults.
The changes in hormone levels that occur during puberty may cause boys and
girls to perceive the world in different ways, leading them to react
differently to situations. Thus, puberty augments behavioral sex differences
between young men and women. In some cultures and religions, puberty is
recognized with rituals that mark the transition into adulthood.
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Adolescence
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Whereas the term puberty
refers to the period of physical maturation, the term adolescence
typically refers to the socially defined period during which a person adjusts
to the physical, emotional, and social changes associated with the transition
from childhood to adulthood. Adolescence, which occurs from about the age of 12
to the age of 17 or older, is a period marked by increased sexual behavior. By
the end of adolescence, two-thirds of young women and almost all young men have
masturbated to orgasm. In recent decades, surveys indicate that more
adolescents have begun engaging in intercourse at a younger age. However,
studies of college students often find that 20 to 30 percent of these students
have not had sexual intercourse. Adolescence can be particularly difficult for
teens who feel different from their peers. Sexually active adolescents may
wonder if their peers are abstinent, while sexually inactive adolescents may
believe that their peers are sexually active. Others may struggle with same-sex
attractions.
Sexual orientation may
become a question during puberty or adolescence. The term sexual orientation
refers to a person's erotic, romantic, or affectional attraction to the other
sex, the same sex, or both. A person who is attracted to the other sex is
labeled heterosexual, or sometimes straight. A person attracted
to the same sex is labeled homosexual. The word gay may be used
to describe homosexuals and is most often applied to men, whereas the term lesbian
is applied to homosexual women. A person who is attracted to both men and women
is labeled bisexual. A transsexual is a person whose sense of
self is not consistent with his or her anatomical sex—for example, a person
whose sense of self is female but who has male genitals. Homosexuality is not
synonymous with transsexuality. Homosexual men's sense of self is male
and lesbian women's sense of self is female.
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Adulthood
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In adulthood, more permanent
relationships, in the form of marriage or cohabitation, become prevalent. The frequency
of sexual activity is different for different individuals. People in monogamous
relationships often engage in sexual activity more frequently than those who
have several partners. It is not unusual for some new couples to have sexual
intercourse almost every day, but in general, among married or cohabiting
couples, the frequency of sexual intercourse tends to decline the longer the
two people are together.
Many individuals remain
sexually active throughout their older years. According to Love, Sex, and
Aging (1984), by American social historian Edward Brecher, a book about sex
among older people in the United States, 33 percent of women 70 years of age
and older and 43 percent of men in the same age range report that they still
masturbate, and 65 percent of married women and 59 percent of married men in
that age range report that they still have sexual intercourse with their
spouses.
As people age, they may
experience physical changes, illnesses, or emotional upheavals, such as the
loss of a partner, that can lead to a decline in sexual interest and behavior.
In women, there is a gradual decline in the function of the ovaries and in the
production of estrogen. The average age at which menopause (the end of
the menstrual cycle) occurs is about 50. Decreased estrogen leads to thinning
of the vaginal walls, shrinking of the vagina and labia majora, and decreased
vaginal lubrication. These conditions can be severe enough to cause the woman
pain during intercourse. Women who were sexually active either through
intercourse or through masturbation before menopause and who continue sexual
activity after menopause are less likely to experience vaginal problems. Women
can use hormone-replacement therapy or hormone-containing creams to help
maintain vaginal health. In men, testosterone production declines over the
years, and the testes become smaller. The volume and force of ejaculation
decrease and sperm count is reduced, but viable sperm may still be produced in
elderly men. Erection takes longer to attain, and the time after orgasm during
which erection cannot occur (the refractory period) increases. Medications and
vascular disease, diabetes, and other medical conditions can cause erectile
dysfunction.
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PHYSIOLOGY
OF SEX
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Understanding the processes
and underlying mechanisms of sexual arousal and orgasm is important to help
people become more familiar with their bodies and their sexual responses and to
assist in the diagnosis and treatment of sexual dysfunctions. Nevertheless, it
was not until the work of American gynecologist William H. Masters and American
psychologist Virginia Johnson that detailed laboratory studies were conducted
on the physiological aspects of sexual arousal and orgasm in a large number of
men and women. Based on data from 312 men and 382 women and observations from
more than 10,000 cycles of sexual arousal and orgasm, Masters and Johnson
described the human sexual response cycle in four stages: excitement, plateau,
orgasm, and resolution.
In men who are unaroused,
the penis is relaxed, or flaccid. In unaroused women, the labia majora lie
close to each other, the labia minora are usually folded over the vaginal
opening, and the walls of the vagina lie against each other like an uninflated
balloon.
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Excitement
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The excitement stage of
sexual arousal is characterized by increased blood flow to blood vessels
(vasocongestion), which causes tissues to swell. In men, the tissues in the
penis become engorged with blood, causing the penis to become larger and erect.
The skin of the scrotum thickens, tension increases in the scrotal sac, and the
scrotum is pulled up closer to the body. Men may also experience nipple
erection.
In women, vasocongestion
occurs in the tissue surrounding the vagina, causing fluids to seep through the
vaginal walls to produce vaginal lubrication. In a process similar to male
erection, the glans of the clitoris becomes larger and harder than usual.
Muscular contraction around the nipples causes them to become erect. However,
as the excitement phase continues, vasocongestion causes the breasts to enlarge
slightly so that sometimes the nipples may not appear erect. Vasocongestion
also causes the labia majora to flatten and spread apart somewhat and the labia
minora to swell and open. The upper two-thirds of the vagina expands in a
“ballooning” response in which the cervix and the uterus pull up, helping to
accommodate the penis during sexual intercourse.
Both women and men may
develop “sex flush” during this or later stages of the sexual response cycle,
although this reaction appears to be more common among women. Sex flush usually
starts on the upper abdomen and spreads to the chest, resembling measles. In
addition, pulse rate and blood pressure increase during the excitement phase.
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Plateau
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During the plateau stage,
vasocongestion peaks and the processes begun in the excitement stage continue
until sufficient tension is built up for orgasm to occur. Breathing rate, pulse
rate, and blood pressure increase. The man's penis becomes completely erect and
the glans swells. Fluid secreted from the Cowper's gland (located near the
urethra, below the prostate) may appear at the tip of the penis. This fluid,
which nourishes the sperm, may contain active sperm capable of impregnating a woman.
In women, the breasts continue to swell, the lower third of the vagina swells,
creating what is called the orgasmic platform, the clitoris retracts into the
body, and the uterus enlarges. As the woman approaches orgasm, the labia majora
darken.
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Orgasm
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Orgasm, or climax, is
an intense and usually pleasurable sensation that occurs at the peak of sexual
arousal and is followed by a drop in sexual tension. Not all sexual arousal
leads to orgasm, and individuals require different conditions and different
types and amounts of stimulation in order to have an orgasm. Orgasm consists of
a series of rhythmic contractions in the genital region and pelvic organs.
Breathing rate, pulse rate, and blood pressure increase dramatically during
orgasm. General muscle contraction may lead to facial contortions and
contractions of muscles in the extremities, back, and buttocks.
In men, orgasm occurs
in two stages. First, the vas deferens, seminal vesicles, and prostate
contract, sending seminal fluid to the bulb at the base of the urethra, and the
man feels a sensation of ejaculatory inevitability—a feeling that ejaculation
is just about to happen and cannot be stopped. Second, the urethral bulb and
penis contract rhythmically, expelling the semen—a process called ejaculation.
For most adult men, orgasm and ejaculation are closely linked, but some men
experience orgasm separately from ejaculation.
In women, orgasm is characterized
by a series of rhythmic muscular contractions of the orgasmic platform and
uterus. These contractions can range in number and intensity. The sensation is
very intense—more intense than the tingling or pleasure that accompany strong
sexual arousal.
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Resolution
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During resolution, the
processes of the excitement and plateau stages reverse, and the bodies of both
women and men return to the unaroused state. The muscle contractions that
occurred during orgasm lead to a reduction in muscular tension and release of
blood from the engorged tissues.
The woman's breasts return
to normal size during resolution. As they do, the nipples may appear erect as
they stand out more than the surrounding breast tissue. Sex flush may disappear
soon after orgasm. The clitoris quickly returns to its normal position and more
gradually begins to shrink to its normal size, and the orgasmic platform
relaxes and starts to shrink. The ballooning of the vagina subsides and the
uterus returns to its normal size. Resolution generally takes from 15 to 30
minutes, but it may take longer, especially if orgasm has not occurred.
In men, erection subsides
rapidly and the penis returns to its normal size. The scrotum and testes shrink
and return to their unaroused position. Men typically enter a refractory
period, during which they are incapable of erection and orgasm. The length of
the refractory period depends on the individual. It may last for only a few
minutes or for as long as 24 hours, and the length generally increases with
age. Women do not appear to have a refractory period and, because of this,
women can have multiple orgasms within a short period of time. Some men also
experience multiple orgasms. This is sometimes related to the ability to have
some orgasms without ejaculation.
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SEXUAL RISKS
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There are a number of
pressing sexually related public health and social policy issues facing
countries around the world today. According to the United States Centers for
Disease Control and Prevention, in the United States a teen becomes pregnant
every 30 seconds, and every 13 seconds a teen contracts a sexually transmitted
infection (STI). For most people in the United States, engaging in heterosexual
intercourse without the use of a condom is the behavior that puts them at
greatest risk for infection with human immunodeficiency virus (HIV), which can
lead to acquired immunodeficiency syndrome (AIDS) and is often ultimately
fatal. Although there is currently no cure for AIDS, there are medications that
can help delay the onset of symptoms. Another serious sexually transmitted
disease is syphilis, which if left untreated for many years, can lead to
paralysis, psychiatric illness, and death. Gonorrhea and chlamydia may produce
no obvious symptoms in a woman, but they can lead to sterility if she is not
treated. Sexually transmitted diseases should be diagnosed and treated by
qualified medical practitioners, and all sexual partners must be treated in
order to avoid reinfection.
Individuals can reduce
their exposure to such sexual risks by practicing abstinence, using appropriate
methods of contraception to avoid unwanted pregnancies, and using safer sex
practices. Such practices include using condoms to avoid exchanging bodily
fluids, limiting the number of sexual partners, and restricting sexual
behaviors to those with less risk, such as manual stimulation and massage.
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SEXUAL
DYSFUNCTIONS
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Sexual dysfunctions are
problems with sexual response that cause distress. Erectile dysfunction
(impotence) refers to the inability of a man to have or maintain an erection. Premature
ejaculation occurs when a man is not able to postpone or control his
ejaculation. Inhibited male orgasm, or retarded ejaculation,
occurs when a man cannot have an orgasm despite being highly aroused.
Female orgasmic dysfunction
(anorgasmia, or inhibited female orgasm) refers to the inability of a woman to
have an orgasm. Orgasmic dysfunction may be primary, meaning that the woman has
never experienced an orgasm; secondary, meaning that the woman has had orgasms
in the past but cannot have them now; or situational, meaning that she has
orgasms in some situations but not in others. Vaginismus refers to a
spastic contraction of the outer third of the vagina, a condition that can
close the entrance of the vagina, preventing intercourse.
Dyspareunia refers to
painful intercourse in either women or men. Low sexual desire is a lack
of interest in sexual activity. Discrepant sexual desire refers to a
condition in which partners have considerably different levels of sexual
interest. These dysfunctions may be caused by physical problems such as fatigue
or illness; the use of prescription medications, other drugs, or alcohol; or
psychological factors, including learned inhibition of sexual response, anxiety,
interfering thoughts, spectatoring (observing and judging one's own
sexual performance), lack of communication between partners, insufficient or
ineffective sexual stimulation, and relationship conflicts. In such cases, a
qualified sex therapist can work with a physician, if necessary, to determine
the cause and best treatment options.
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STUDIES
OF HUMAN SEXUALITY
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Sexuality and lovemaking
techniques have been studied in various cultures since ancient times. The Kama
Sutra, written in India in the 2nd century bc,
is one of the best-known ancient sex manuals. It discusses the spiritual
aspects of sexuality and presents many sexual positions and techniques for
enhancing enjoyment of intercourse.
In Europe and the United
States, the scientific study of human sexuality began in the late 19th century
during the Victorian Age, a time of repressive sexual norms. German
psychiatrist Richard von Krafft-Ebing focused on what he considered to be the
psychopathological problems of sex. Viennese physician Sigmund Freud, founder
of psychoanalysis, considered sexuality central to his psychoanalytic theory.
Havelock Ellis, an English physician, collected a wealth of information on
sexuality from case histories, medical research, and anthropological reports.
The first work in his series Studies in the Psychology of Sex was
published in 1896. His scientific objectivity foreshadowed modern sexology.
Early in the 20th century, German physician Magnus Hirshfeld founded the first
sex-research institute in Germany. He conducted the first large-scale sex
survey, collecting data from 10,000 men and women. He also initiated the first
journal for publishing the results of sex studies, and started a
marriage-counseling service. Most of his materials were destroyed by the Nazis
during World War II (1939-1945).
In the early 1930s, American
anthropologist Margaret Mead and British anthropologist Bronislaw Malinowski
began collecting data on sexual behavior in other cultures. The most noted
scientific studies of sexuality in the 20th century are those of American
biologist Alfred Charles Kinsey and his colleagues and those of William H.
Masters and Virginia Johnson. Kinsey began interviewing people about their
sexual histories in 1938, and with his colleagues he published Sexual
Behavior in the Human Male (1948) and Sexual Behavior in the Human
Female (1953), based mostly on interviews with 5,300 white men and 5,940
white women. Masters and Johnson began their clinical studies of the physiology
of sexual response and sexual dysfunctions in the 1950s. These observations
were published in Human Sexual Response (1966) and Human Sexual
Inadequacy (1970), among others. Smaller studies have confirmed many of the
findings of these pioneering sex researchers and have challenged certain
others. The AIDS crisis has prompted a number of contemporary surveys of sex,
including the National Health and Social Life Survey, the results of which were
published in the book Sex in America (1994). As in any area of science,
particularly relatively new and sensitive areas such as sex research, these
studies have been criticized, on the basis of their findings and methodologies,
but each study brings us closer to a fuller understanding of human sexuality.
