Introduction Sex hormones exert an important influence on behavior. These hormones control sexual maturation at puberty, and they have an impact on the sex drive and on sexual activity throughout the life span. In most animal species, sex hormones completely control sexual behavior. In humans, their role is more limited. Human sexual motivation is the result of the complex interplay of hormones, psychological factors, and social factors.
At puberty, the brain releases several hormones that travel through the bloodstream to target organs. In males, the targets are the testes; in females, the ovaries. In response to the hormones released from the brain, these targets begin to produce the sex hormones. The principal male sex hormone is testosterone
(the name refers to the fact that it is produced by the testes). With the production of testosterone at puberty in human males, facial and body hair grows, bones and muscles develop more fully, the voice deepens, and the genitals enlarge. In females, two sex hormones are produced by the ovaries at puberty, estradiol
(often called estrogen) and progesterone. Estradiol is responsible for breast development, changes in female appearance (for example, wider hips), and maturation of the genitals and uterus. Progesterone plays a major role in preparing the female body for menstruation and pregnancy. It should be noted that although testosterone is called the “male sex hormone,” it is also found in females, though in much smaller quantities. The same applies to the female sex hormones, which are found in males.
After puberty is completed, the sex hormones continue to play a role in sexual motivation. In most mammal species, the female goes through a regular cycle, the estrous cycle, during which sex hormones are released, causing an increase in sex drive. During this phase of the cycle, a female will seek a male partner with the goal of reproduction; she is said to be “in heat.” In the presence of a female in estrus, the male experiences an increase in the sex hormone testosterone, and he will be sexually attracted to the female. For example, male rats will ignore a female whose ovaries have been removed and who thus cannot produce estradiol; however, an injection of estradiol will make the female interested in sex, and males will approach her for sexual activity. In these animals, sexual behavior is largely determined by the females’ sexual readiness, which depends on the phase of estrus.
In most animal species, the male learns of a female’s sexual readiness by the presence of a chemical she emits called a pheromone. Pheromones are usually derived from vaginal secretions during estrus. If pheromones are collected from an animal in estrus and applied to another that is not in estrus, males will approach the nonreceptive female and attempt sexual intercourse. In humans, pheromones do not appear to play a significant role in sexual attraction.
The Human Condition Humans are quite different from other animals in other respects. While the sex hormones do have an important influence, they do not control sexual motivation and behavior. Research has shown that males who have abnormally low levels of testosterone have problems achieving erection and often have a very low sex drive. Injections of testosterone restore the ability to obtain normal erection in these cases. Abnormally high levels of testosterone, however, do not cause an unusually high sex drive. Apparently, there is a minimum level of testosterone necessary for normal sexual behavior, but higher levels do not seem to have any significant effect.
Like females of other animal species, the human female goes through a cycle during which levels of sex hormones are increased. This cycle in humans and primates is called the menstrual cycle. Although human females experience cyclic increases in the sex hormones estradiol and progesterone, there is no clear indication of change in sexual motivation during this phase. For most women, sex drive does not vary with sex hormone levels. For example, women who have had their ovaries removed continue to experience a normal sex drive. The same usually applies to women who have undergone menopause, the stage of the life in the late forties during which the ovaries cease producing sex hormones. Most postmenopausal women continue to enjoy a normal sex life in spite of their reduced sex hormone levels.
Though the role of the sex hormones in sexual motivation in women is limited, it appears that testosterone plays a role. In women, small amounts of testosterone are produced by the adrenal glands, small glands that secrete several hormones and are located atop the kidneys. Removal of the adrenal glands often abolishes the sex drive in women. In adult female monkeys whose adrenal glands and ovaries have been removed, injections of testosterone restore sex drive and sexual activity. In women, while testosterone appears to play a role, many psychological factors such as attitudes and religious beliefs seem to be more important in understanding sexual motivation.
Hormones and Sexual Dysfunctions Knowledge about sex hormones and their role in sexual motivation has been used in several ways. With the discovery that castration, or removal of the testes, drastically lowers sex drive, it was concluded that some cases of impaired sex drive in males may be caused by low levels of testosterone. For example, a soldier was castrated by an explosive during World War I. As a result, he lost interest in sex, was unable to obtain an erection, and lost significant muscle mass; his hips also expanded. After receiving five injections of high doses of testosterone, he was experiencing normal erections, his sex drive returned, and he began to gain weight and muscle mass. For males whose sexual difficulties are caused by abnormally low levels of testosterone, regular injections tend to restore a normal sex drive and the ability to achieve erections. Studies reveal, however, that not all individuals who are castrated experience changes in sexual behavior. For reasons that are unclear, some males appear capable of sexual behavior in spite of the removal of the testes.
Giving doses of estradiol or progesterone to men has several effects. First, excesses of these hormones cause a sharp decline in the production of testosterone, which may interfere with sex drive, erection, and ejaculation. Administering estradiol also causes breast enlargement in men. When given to men, progesterone causes a decrease in sex drive and erection ability. It does not tend to make males develop a female appearance, as estradiol does. This knowledge has been applied to treating sex offenders. Compulsive sex offenders, especially child molesters, are sometimes given injections of progesterone to help them control their sexual urges. When combined with counseling, this treatment may be useful in helping these persons gain some control over their sexual activity. While receiving the injections, the sex offenders often lose their sex drive and have trouble achieving erections. These effects are only temporary and disappear when progesterone is no longer given.
Women with unusually low levels of estradiol do not appear to suffer from direct sexual problems. They do experience difficulties with vaginal lubrication. Vaginal dryness makes sexual intercourse difficult and painful as a result of friction. One remedy for this problem is the use of a lubricant.
One early theory of homosexuality
proposed that an imbalance in the sex hormones was responsible for sexual attraction to members of the same sex. According to the theory, male homosexuality was caused by a deficiency in testosterone or an excess of estradiol. This would explain why some men display the behavior expected of women: attraction to men and, according to popular stereotypes, effeminate mannerisms. Female homosexuality was believed to be caused by excess testosterone and, possibly, insufficient estradiol. Scientific tests of the theory have consistently failed to support it. Homosexuals, both male and female, do not differ from heterosexuals in their circulating sex hormone levels. Therefore, the hormone imbalance theory of homosexuality is no longer accepted. In fact, abnormally low levels of testosterone in men do not make them sexually attracted to men; the typical result is a decrease in sexual interest for any type of sexual partner. In women, excessively elevated levels of testosterone do not cause lesbianism; if anything, they tend to increase women’s sexual interest in men. Factors other than levels of sex hormones are important in understanding homosexuality.
Another application involves the treatment of transgender individuals—those whose gender identity does not match the one they were assigned at birth based on their anatomy. Sex hormones may be used in hormone replacement therapy for people who want to bring their secondary sex characteristics more into line with their gender identity. For example, transgender women—those who identify as female although they were born with a male anatomy—may be administered regular doses of estrogens. As a result, the person will experience breast growth, smoother skin, loss of muscle mass, and decreases in erection and sperm production. The treatment does not change facial and body hair or raise the pitch of the voice, but physical appearance will become more femalelike. For transgender men—those who identify as male although they were born with a female body—testosterone is administered, which leads to growth of facial and body hair, deepening of the voice, and the end of menstruation. Hormone therapy is often a precursor to gender reassignment surgery for transgender individuals.
Instincts and Culture The term “hormone” is derived from the Greek hormaein, which means “to set in motion.” It was first used in 1904 to refer to those bodily substances that can have a profound influence on human development and behavior. All knowledge about the sex hormones is relatively new. Undoubtedly, much more will be discovered as medical technology continues to develop rapidly.
Knowledge about the effects of castration dates back to ancient China and Arabic countries. In these countries castration was practiced to provide safe guardians of the royal harem. The castrated guardians, or eunuchs, were considered safe since their sexual motivation was impaired as a result of testosterone deficiency. In Europe, up to the beginning of the nineteenth century, boys in church choirs were sometimes castrated to ensure their continued ability to sing soprano.
Thus, a general understanding about the importance of the testes in affecting human development and behavior is not new. Information about the exact nature and role of sex hormones, however, is relatively new. For example, the eighteenth century physician Simon Tissot believed that the results of castration were caused by impairments in semen production. According to his view, the loss of semen caused a decrease in strength, passivity, and a variety of other complications. It was not until the twentieth century that the loss of testosterone was identified as the mechanism underlying the effects of castration.
Although sexual motivation in lower animals is readily understood in terms of sex hormones and instincts, human sexual behavior is much more complex. A number of cultural and psychological factors, in combination with sex hormones, interact to determine human sexual motivation and behavior. The range of sexual activities in lower animals is limited and rigidly controlled by the phase of the female’s estrous cycle. In humans, there is tremendous variability in types of sexual behavior, timing and frequency of sex, choice of partners, number of partners, and context of sexual activity. These variables are influenced by cultural standards regarding sexual activity. In permissive cultures, such as in Polynesia, sexual experimentation is encouraged and expected. Other cultures are more restrictive and discourage sex before marriage.
Within each culture, other factors such as peer group influence, familial and religious beliefs and values toward sexuality, and individual fears (such as pregnancy and sexually transmitted diseases) can also have an impact on human sexual behavior. For example, the threat of acquired immunodeficiency syndrome (AIDS) has led some individuals to change their sexual behavior. Abstinence and monogamy are advocated by some, while others, including many adolescents, have adopted few changes. On the other hand, the development of drugs such as sildenafil (Viagra) that overcome the effects of the normal decline of male sex hormones with age is part of a movement to increase conscious control of and prolongation of sexuality.
Thus, although sex hormones play an important role in human sexual behavior, especially with respect to sexual maturity and sex drive, several cultural and psychological influences are more important. To address the problems of teenage pregnancy and sexually transmitted disease, including AIDS, all factors that determine human sexual motivation and behavior must be explored in depth.
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