The Biology of Sex

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Chapter 8 Motivation and Emotion
■ What do we still need to know?
Many questions remain. The Chicago survey did not ask about some of the more controversial aspects of sexuality, such as the effects of pornography or the role in sexual
activity of sexual fetishes such as shoes or other clothing. Had the researchers asked
about such topics, their results might have painted a different picture. Further, because
the Chicago survey focused on people in the United States, it told us little or nothing
about the sexual practices, traditions, and values of people in the rest of the world.
The Chicago team has continued to conduct interviews, and the results are beginning to fill in the picture about sexual behavior in the United States and around the
world (Youm & Laumann, 2002). They have found, for example, that nearly one-quarter
of U.S. women prefer to achieve sexual satisfaction without partners of either sex. And,
although people in the United States tend to engage in a wider variety of sexual behaviors than do those in Britain, people in the United States appear to be less tolerant of
disapproved sexual practices (Laumann & Michael, 2000; Michael et al., 1998). Other
researchers have found a number of consistent gender differences in sexuality. For
example, men tend to have a stronger interest in and desire for sex than women,
whereas women are more likely than men to associate sexual activity with a committed relationship (Baumeister, Catanese, & Vohs, 2001; Peplau, 2003; Regan & Berscheid,
1999).
The results of even the best survey methods—like those of the best of all other
research methods—usually raise as many questions as they answer. When do people
become interested in sex, and why? How do they choose to express these desires, and
why? What determines their sexual likes and dislikes? How do learning and sociocultural factors modify the biological forces that seem to provide the raw material of
human sexual motivation? These are some of the questions about human sexual behavior that a survey cannot easily or accurately explore (Benson, 2003).
The Biology of Sex
sexual response cycle The pattern of
arousal before, during, and after sexual
activity.
sex hormones Chemicals in the blood
that organize and motivate sexual
behavior.
estrogens Feminine hormones that
circulate in the bloodstream.
progestins Feminine hormones that
circulate in the bloodstream.
androgens Masculine hormones that
circulate in the bloodstream.
Observations in Masters and Johnson’s laboratory led to important findings about the
sexual response cycle, the pattern of physiological arousal before, during, and after
sexual activity (see Figure 8.3). Masters and Johnson (1966) found that men show one
primary pattern of sexual response and that women display at least three different patterns from time to time. In both men and women, the first, or excitement, phase begins
with sexually stimulating input from the environment or from one’s own thoughts. Further stimulation leads to intensified excitement in the second, or plateau, phase. If stimulation continues, the person reaches the third, or orgasmic, stage. Although orgasm
lasts only a few seconds, it provides an intensely pleasurable release of physical and psychological tension. The resolution phase follows, during which the person returns to a
state of relaxation. At this point, men enter a refractory period, during which they are
temporarily unable to be aroused. Women are capable of immediately repeating the
cycle if stimulation continues.
People’s motivation to engage in sexual activity has biological roots in sex hormones. The female sex hormones are estrogens and progestins; the main ones are
estradiol and progesterone. The male hormones are androgens; the principal example
is testosterone. Each sex hormone flows in the blood of both sexes, but males have relatively more androgens, and women have relatively more estrogens and progestins.
Sex hormones have both organizing and activating effects on the brain. The organizing effects are permanent changes in the brain that influence the brain’s response to
hormones. The activating effects are temporary behavioral changes that last only as long
as a sex hormone’s level is elevated—such as in the ovulation phase of the monthly
menstrual cycle. In mammals, including humans, the organizing effects of hormones
occur around the time of birth. It is then that certain brain areas are sculpted into a
“female-like” or “male-like” pattern. For example, a brain area called BnST is generally