Essay Grade

Bruce Jenner spent his early career as an athlete, playing football at Graceland College until a
knee injury necessitated a change in sport to decathlon in 1968. After years of grueling training
and competition, Jenner won a gold medal in decathlon at the 1976 Summer Olympics in
Montreal and became a hero back home in the United States. Jenner even graced the cover of
Sports Illustrated magazine and became the most widely known athlete to be on the cover of a
Wheaties breakfast cereal box. Leaving athletics behind, he became better known as a sports
commentator and occasional actor in films and television. During this time he had been married
three times and fathered six children. Throughout the successes in athletics and as an actor,
Jenner was struggling with a psychological disorder called gender dysphoria, which is the
distress that people feel when their gender identity does not match their sex at birth. At times he
dressed as a woman, and he was taking female hormones to try to better match his feelings of
being female.
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In April 2015, Jenner made news by coming out as a transgendered woman. Later that year,
Jenner officially adopted her now-permanent feminine identity as Caitlyn Jenner, and in January
2017, she underwent gender reassignment surgery to replace the penis with a vaginal opening.
Her memoir The Secrets of My Life and the documentary series I Am Cait detailed her gender
transition; in recognition of her outspoken support for LGBT rights and the strength she
demonstrated in discussing her gender identity struggles with the public, she received the Arthur
Ashe Courage Award and Out magazine’s Newsmaker of the Year award. Quite possibly the
most interesting thing about Caitlyn Jenner’s new gender and identity is that she still finds
herself sexually attracted to women, underscoring the fact that gender identity does not always
match a person’s sexual orientation.
Source: Valerie Macon/AFP/Getty Images.
Humans have a great affinity for dichotomies, dividing their world into blacks and whites with
few grays in between. No dichotomy is more significant for human existence than that of male
and female: One’s sex is often the basis for deciding how the person should behave, what the
person can do, and with whom the person should fall in love. Not only are many of the
differences between males and females imposed on them by society, but Caitlyn’s experience
suggests that typing people as male or female may not be as simple or as appropriate as we think.
We will encounter even more puzzling cases later as we take a critical look at the designation of
male versus female and the expectations that go with it. In the meantime, we need to continue
our discussion of motivation by considering how sex is like and unlike other drives.
Sex as a Form of Motivation
To say that sex is a motivated behavior like hunger may be stating the obvious. But theorists
have had difficulty categorizing sex with other physiological drives because it does not fit the
pattern of a homeostatic tissue need. If you fail to eat or if you cannot maintain body temperature
within reasonable limits, you will die. But no harm will come from forgoing sex; sex ensures the
survival of the species but not of the individual.
There are, however, several similarities with other drives like hunger and thirst. They include
arousal and satiation, the involvement of hormones, and control by specific areas in the brain.
We will explore these similarities as well as some differences in the following pages.
Arousal and Satiation
The cycle of arousal and satiation is the most obvious similarity between sexual motivation and
other motivated behaviors such as hunger and thirst. In the 1960s, William Masters and Virginia
Johnson conducted groundbreaking research on the human sexual response. Until then, research
had been limited to observing sexual behavior in animals or interviewing humans about their
sexual activity. Masters and Johnson (1966) observed 312 men and 382 women and recorded
their physiological responses during 10,000 episodes of sexual activity in the laboratory. This
kind of research was unheard of at the time; in fact, the researchers had trouble finding journals
that would publish their work. Their work on human sexual behavior was the subject of the
recent Showtime cable series Masters of Sex.
How is sex like and unlike other drives?
Masters and Johnson identified four phases of sexual response (Figure 7.1). The excitement
phase is a period of arousal and preparation for intercourse. Both sexes experience increased
heart rate, respiration rate, blood pressure, and muscle tension. The male’s penis becomes
engorged with blood and becomes erect. The female’s clitoris becomes erect as well, her vaginal
lips swell and open, the vagina lubricates, her breasts enlarge, and the nipples become erect.
While hunger is mostly a function of time since the last meal, sexual arousal is more influenced
by opportunity and sexual stimuli such as explicit conversation or the presence of an attractive
person. In contrast to humans, sexual arousal in most mammal species is triggered by a surge in
hormones. Another difference between sex and other drives is that we usually are motivated to
reduce hunger, thirst, and temperature deviations, but we seek sexual arousal. This difference is
not unique, though; for example, we might skip lunch to increase the enjoyment of a gourmet
dinner.
During the plateau phase, the increase in sexual arousal levels off. Arousal is maintained at a
high level for seconds or minutes, though it is possible to prolong this period. The testes rise in
the scrotum in preparation for ejaculation; vaginal lubrication increases and the vaginal entrance
tightens on the penis. During orgasm, rhythmic contractions in the penis are accompanied by
ejaculation of seminal fluid containing sperm into the vagina. Similar contractions occur in the
vagina. This period lasts just a few seconds but involves an intense experience of pleasure.
Orgasm is similar to the pleasure one feels after eating or when warmed after a deep chill, but it
is unique in its intensity; the resolution that follows is reminiscent of the period of quiet
following return to homeostasis with other drives.
After orgasm, males have a refractory phase, during which they are unable to become aroused or
have another orgasm for minutes, hours, or even days, depending on the individual and the
circumstances. Females do not experience a refractory period and can have additional orgasms
anytime during the resolution phase. When comparing the sex drive with other kinds of
motivation, the male refractory period has an interesting parallel with sensory-specific satiety
(see Chapter 6); it is called the Coolidge effect. According to a popular but probably
questionable story, President Coolidge and his wife were touring a farm when Mrs. Coolidge
asked the farmer whether the flurry of sexual activity among the chickens was the work of one
rooster. The farmer answered yes, that the rooster copulated dozens of times each day, and Mrs.
Coolidge said, “You might point that out to Mr. Coolidge.” President Coolidge, so the story
goes, then asked the farmer, “Is it a different hen each time?” The answer again was yes. “Tell
that to Mrs. Coolidge,” the president replied. Whether the story is true or not, the Coolidge
effect—a quicker return to sexual arousal when a new partner is introduced—has been
observed in a wide variety of species. We will visit the subject again shortly.
Figure 7.1 Phases of the Sexual Response Cycle.
Source: From Psychology: The Adaptive Mind (2nd ed.), by J. S. Nairne, 2000, Wadsworth, a
part of Cengage Learning, Inc.
The Role of Testosterone
As important as sex is to humans, it is ironic that so much of what we know about the topic
comes from the study of other species. One reason is that research into human sexual behavior
was for a long time considered off-limits, and funding was hard to come by. Another reason is
that sexual behavior is more “accessible” in nonhuman animals; rats have sex as often as 20
times a day and are not at all embarrassed to perform in front of the experimenter. In addition,
we can manipulate their sexual behavior in ways that would be considered unethical with
humans. Hormonal control in particular is more often studied in animals because hormones have
a clearer role in animal sexual behavior.
Castration, or removal of the gonads (testes or ovaries), is one technique used to study
hormonal effects because it removes the major source of sex hormones; castration results in a
loss of sexual motivation in nonhuman mammals of both sexes. Sexual behavior may not
disappear completely, because the adrenal glands continue to produce both male and female
hormones, though at a lesser rate than the gonads. The time course of the decline is also variable,
ranging from a few weeks to five months in male rats (J. M. Davidson, 1966); across several
species, animals who are sexually experienced are impaired the least and decline the slowest (B.
Hart, 1968; Sachs & Meisel, 1994). Humans are less at the mercy of fluctuating hormone levels
than other animals, but when they are castrated (usually for medical reasons, such as cancer),
sexual interest and functioning decrease in both males and females (Bremer, 1959; Heim, 1981;
Sherwin & Gelfand, 1987; Shifren et al., 2000). The decline takes longer in humans than in rats,
but the rate is similarly variable.
Castration has been elected by some male criminals in the hope of controlling aggression or
sexual predation, sometimes in exchange for shorter prison sentences. Castration is an extreme
therapy; drugs that counter the effects of androgens (a class of hormones responsible for a
number of male characteristics and functions) are a more attractive alternative. Those that
block the production of the androgen testosterone, the major sex hormone in males, have been
80%–100% effective in eliminating deviant sexual behavior such as exhibitionism and
pedophilia (sexual contact with children), along with sexual fantasies and urges (A. Rösler &
Witztum, 1998; Thibaut, Cordier, & Kuhn, 1996). Chemical castration is either allowed on a
voluntary basis or mandated for some offenses in nine states in the United States (M. Park, 2012)
and in several other countries. The effects of castration indicate that testosterone is necessary for
male sexual behavior, but the amount of testosterone required appears to be minimal; men with
very low testosterone levels can be as sexually active as other men (Raboch & Stárka, 1973).
Frequency of sexual activity does vary with testosterone levels within an individual, but the
testosterone increases appear to be the result of sexual activity rather than the cause. For
example, testosterone levels are high in males at the end of a period in which intercourse
occurred, not necessarily before (J. M. Dabbs & Mohammed, 1992; Kraemer et al., 1976). A
case report (which is anecdotal and does not permit us to draw conclusions) suggests that just the
anticipation of sex can increase the testosterone level. Knowing that beard growth is related to
testosterone level, a researcher working in near isolation on a remote island weighed the daily
clippings from his electric razor. He found that the amount of beard growth increased just before
planned visits to the mainland and the opportunity for sexual activity (Anonymous, 1970).
Figure 7.2 Female-Initiated Activity During the Menstrual Cycle.
Source: From figure 2 from “Rise in Female-Initiated Sexual Activity at Ovulation and Its
Suppression by Oral Contraceptives,” by D. B. Adams, A. R. Gold, and A. D. Burt, 1978, New
England Journal of Medicine, 299(21), pp. 1145–1150.
In most mammals, females are unwilling to engage in sex except during estrus, a period when
the female is ovulating, sex hormone levels are high, and the animal is said to be in heat.
Human females and females of some other primate species engage in sex throughout the
reproductive cycle. Studies of sexual frequency in women have not shown a clear peak at the
time of ovulation. However, initiation of sex is a better gauge of the female’s sexual motivation
than is her willingness to have sex; women do initiate sexual activity more often during the
middle of the menstrual cycle, which is when ovulation occurs (Figure 7.2; D. B. Adams, Gold,
& Burt, 1978; S. M. Harvey, 1987). The researchers attributed the effect to estrogen, a class of
hormones responsible for a number of female characteristics and functions. Their reasons
were that estrogen peaks at midcycle and the women did not increase in sexual activity if they
were taking birth control pills, which level out estrogen release over the cycle.
However, testosterone peaks at the same time, and the frequency of intercourse during midcycle
corresponds to the woman’s testosterone level (N. M. Morris, Udry, Khan-Dawood, & Dawood,
1987). At menopause, when both estrogen and testosterone levels decline, testosterone levels
show the most consistent relationship with intercourse frequency (McCoy & Davidson, 1985).
How to interpret these observations is unclear, because testosterone increases in women as a
result of sexual activity, just as it does in men (Figure 7.3; J. M. Dabbs & Mohammed, 1992).
However, studies in which testosterone level was manipulated demonstrate that it also
contributes to women’s sexual behavior. Giving a dose of testosterone to women increases their
arousal during an erotic film (Tuiten et al., 2000). More important, in women who had their
ovaries removed, testosterone treatment increased sexual arousal, sexual fantasies, and
intercourse frequency, but estrogen treatment did not (Sherwin & Gelfand, 1987; Shifren et al.,
2000).
Figure 7.3 Relationship Between Sexual Behavior and Salivary Testosterone Levels in Men and
Women.
Source: From “Male and Female Salivary Testosterone Concentrations Before and After Sexual
Activity,” by J. M. Dabbs, Jr. and S. Mohammed, Physiology and Behavior, 52, pp. 195–197,
Fig. 1. © 1992 Reprinted with permission from Elsevier Science.
Brain Structures and Neurotransmitters
As neuroscientists developed a clearer understanding of the roles of various brain structures,
motivation researchers began to shift their focus from drive as a tissue need to drive as a
condition in particular parts of the brain. Sexual activity, like other drives and behaviors,
involves a network of brain structures. This almost seems inevitable, because sexual activity
involves reaction to a variety of stimuli, activation of several physiological systems, postural and
movement responses, a reward experience, and so on. We do not understand yet how the sexual
network operates as a whole, but we do know something about the functioning of several of its
components. In this section, you will see some familiar terms, the names of hypothalamic
structures you learned about in Chapter 6. This illustrates an important principle of brain
functioning, that a particular brain area, even a very small one, often has multiple functions.
What is the role of testosterone in sexual behavior?
Two areas are important in sexual behavior in both sexes, the medial preoptic area of the
hypothalamus and the medial amygdala. The medial preoptic area (MPOA) is one of the more
significant brain structures involved in male and female sexual behavior. (Be careful not to
confuse the medial preoptic area with the median preoptic nucleus, discussed in Chapter 6. They
are both in the preoptic area, which you can locate in Figure 6.2.) Stimulation of the MPOA
increases copulation in rats of both sexes (Bloch, Butler, & Kohlert, 1996; Bloch, Butler,
Kohlert, & Bloch, 1993), and the MPOA is active when they copulate spontaneously (Pfaus,
Kleopoulos, Mobbs, Gibbs, & Pfaff, 1993; Shimura & Shimokochi, 1990). The MPOA appears
to be more responsible for performance than for sexual motivation; when it was destroyed in
male monkeys, they no longer tried to copulate, but instead they would often masturbate in the
presence of a female (Slimp, Hart, & Goy, 1978).
What brain structures are involved in sexual behavior?
The medial amygdala also contributes to sexual behavior in rats of both sexes. Located near
the lateral ventricle in each temporal lobe, the amygdala is involved not only in sexual
behavior but also in aggression and emotions. The medial amygdala is active while rats
copulate (Pfaus et al., 1993), and stimulation causes the release of dopamine in the MPOA
(Dominguez & Hull, 2001; Matuszewich, Lorrain, & Hull, 2000). The medial amygdala’s role
apparently is to respond to sexually exciting stimuli, such as the presence of a potential sex
partner (de Jonge, Oldenburger, Louwerse, & Van de Poll, 1992).
Figure 7.4 The Sexually Dimorphic Nuclei of the Rat.
Source: From “The Neuroendocrine Regulation of Sexual Behavior,” by R. A. Gorski, pp. 1–58,
in G. Newton and A. H. Riesen (Eds.) Advances in Psychobiology (Vol. 2), 1974, New York:
Wiley. Reprinted with permission of John Wiley & Sons, Inc.
There are other areas that are involved in sexual behavior but only in the behaviors of a single
sex. Especially significant for males is the sexually dimorphic nucleus (SDN), located in the
preoptic area (de Jonge et al., 1989). The SDN is three to four times larger in male rats
than in females (Figure 7.4; He, Ferguson, Cui, Greenfield, & Paule, 2013), and a male’s
level of sexual activity is related to the size of the SDN, which in turn depends, at least in part,
on protection by testosterone from the cell death that occurs during the pruning stage shortly
after birth (He et al., 2013). Destruction of the SDN reduces male sexual activity (de Jonge et al.,
1989). The SDN’s connections to other sex-related areas of the brain suggest that it integrates
sensory and hormonal information and coordinates behavioral and physiological responses to
sensory cues (Roselli, Larkin, Resko, Stellflug, & Stormshak, 2004). Two other hypothalamic
structures are also important. The paraventricular nucleus (PVN; see Figure 6.2) is important for
male sexual performance and, particularly, for penile erections (Argiolas, 1999). The
ventromedial hypothalamus is active in females during copulation (Pfaus et al., 1993), and
its destruction reduces the female’s responsiveness to a male’s advances (Pfaff & Sakuma,
1979).
Figure 7.5 Dopamine Levels in the Nucleus Accumbens During the Coolidge Effect.
Source: From “Dynamic Changes in Nucleus Accumbens Dopamine Efflux During the Coolidge
Effect in Male Rats,” by D. F. Fiorino, A. Coury, and A. G. Phillips, 1997, Journal of
Neuroscience, 17, p. 4852. © 1997 Society for Neuroscience. Reprinted with permission.
For obvious reasons, we know much less about the brain structures involved in human sexual
behavior. Functional MRI (fMRI) recording during masturbation has confirmed the involvement
of the medial amygdala and PVN in human sexual activity (Komisaruk et al., 2004). PVN
neurons are known to secrete the hormone/neuromodulator oxytocin, which contributes to
male and female orgasm and the intensity of its pleasure (Carmichael, Warburton, Dixen, &
Davidson, 1994). We will see additional results from human research in the discussion of
neurotransmitters. We also know that a few brain structures in humans differ in size between
males and females. Because their contribution to sexual behavior is not clear and the size
differences may also distinguish homosexuals from heterosexuals, we will defer discussion of
these structures until we take up the subject of sexual orientation.
Sexual behavior involves several neurotransmitters, but dopamine has received the most
attention. You saw in Chapter 5 that dopamine level increases in the nucleus accumbens during
sexual activity, and in this chapter that stimulation of the medial amygdala releases dopamine in
the MPOA. Dopamine activity in the MPOA contributes to sexual motivation in males and
females of several species (E. M. Hull et al., 1999). In males, dopamine is critical for sexual
performance: Initially, it stimulates D1 receptors, activating the parasympathetic system and
increasing motivation and erection of sexual tissues; as dopamine level increases, it activates D2
receptors, which shifts autonomic balance to the sympathetic system, resulting in orgasm and
ejaculation. D2 activity also inhibits erection, which probably accounts in part for the sexual
refractory period in males. Interestingly, dopamine release parallels sexual behavior during the
Coolidge effect. As you can see in Figure 7.5, it increased in the male rat’s nucleus accumbens in
the presence of a female, dropped back to baseline as interest waned, and then increased again
with a new female (Fiorino, Coury, & Phillips, 1997). The changes occurred even when the male
and female were separated by a clear panel, so the dopamine level reflects the male’s interest
rather than the effects of sexual behaviors.
Our knowledge about the role of dopamine in human sexual behavior is less precise but
nevertheless intriguing. Drugs that increase dopamine levels, such as those used in treating
Parkinson’s disease and stimulants, increase sexual activity in humans (Evans, Haney, & Foltin,
2002; Meston & Frolich, 2000). The dopamine system has been reported to be active in the
ventral tegmental area in males during ejaculation (Holstege et al., 2003) and in the nucleus
accumbens in females during orgasm (Komisaruk et al., 2004). This activity likely reflects a
reward response, but, significantly, the activated areas also have strong motor output to the
pelvic floor muscles, which are important in orgasmic activity. Variations in the gene for the D4
receptor (DRD4) are associated with sexual arousal and functioning (Ben Zion et al., 2006), and
one variant is correlated with promiscuity and sexual infidelity (Garcia et al., 2010).
Ejaculation is also accompanied by serotonin increases in the lateral hypothalamus, which
apparently contributes further to the refractory period (E. M. Hull et al., 1999). Injecting a drug
that inhibits serotonin reuptake into the lateral hypothalamus increases the length of time before
male rats will attempt to copulate again and their ability to ejaculate when they do return to
sexual activity. Humans take serotonin reuptake blockers to treat anxiety and depression, and
they often complain that the drugs interfere with their ability to have orgasms. The antianxiety
drug buspirone, by contrast, decreases the release of serotonin and facilitates orgasms
(Komisaruk, Beyer, & Whipple, 2008).
An interesting model for the regulation of gender-related aggressive and bonding behaviors has
been proposed in the steroid/peptide theory of social bonds (van Anders, Goldey, & Kuo, 2011).
According to this theory, the balance among testosterone, oxytocin, and vasopressin determine
behaviors such as aggression and intimacy (Figure 7.6). As you probably guessed, a high
testosterone level in either sex increases aggression, but it also impairs the formation of close
social bonds. Oxytocin (involved in muscle contractions of sexual tissue and in social bonding)
and vasopressin (a potent neuromodulator of brain activity) modulate the form of intimacy and
aggression. Antagonistic aggression (which includes social dominance, partner acquisition, and
defense of partners and territory) is seen in those with low levels of vasopressin, whereas
protective aggression (such as defending children or partners) is seen in those with high levels of
vasopressin. Intimacy increases oxytocin, but its interaction with testosterone levels determines
whether that intimacy is sexual (if testosterone is high) or nurturing (if testosterone is low).
Therefore, testosterone levels determine the relative amount of competitive versus nurturing
behaviors an individual expresses, whereas oxytocin determines the relative amount of social
bonding versus social isolation.
Figure 7.6 The Steroid/Peptide Theory of Social Bonds.
Source: From “The Steroid/Peptide Theory of Social Bonds: Integrating testosterone and peptide
responses for classifying social behavioral contexts,” by S. M. van Anders, K. L. Goldey, & P.
X. Kuo. Psychoneuroendocrinology, 36(9). © Elsevier. Reprinted with permission.
Odors, Pheromones, and Sexual Attraction
Sexual behavior results from the interplay of internal conditions, particularly hormone levels,
with external stimuli. Sexual stimuli can be anything from brightly colored plumage or an
attractive body shape to particular odors. Here we will examine the role of odors and
pheromones in sexual attraction, with emphasis on how important they might be for humans.
Before we launch into this discussion, we need to have a basic understanding of the olfactory
(smell) system. Olfaction is one of the two chemical senses, along with taste. Airborne odorous
materials entering the nasal cavity must dissolve in the mucous layer overlying the receptor cells;
the odorant then stimulates a receptor cell when it comes in contact with receptor sites on the
cell’s dendrites (Figure 7.7). Axons from the olfactory receptors pass through openings in the
base of the skull to enter the olfactory bulbs, which lie over the nasal cavity. From there, neurons
follow the olfactory nerves to the nearby olfactory cortex tucked into the inner surfaces of the
temporal lobes.
Figure 7.7 The Olfactory and Vomeronasal Systems.
By varying the number of components in odor mixtures, researchers have calculated that humans
can distinguish a trillion odors (Bushdid, Magnasco, Vosshall, & Keller, 2014). But we do not
have a different receptor for each odor, and an individual neuron cannot produce the variety of
signals required to distinguish among so many different stimuli. Researchers have discovered
that humans have around 400 different receptor genes that produce an equal number of receptor
types, but additional alleles of some of these genes brings the total to about 600 (Olender et al.,
2012). Variation in these alleles among individuals suggests considerable variation in what
different people can smell. We’re pikers compared with dogs (800); mice (1,100); and the
African elephant, which has 2,000 different receptor genes (Niimura, Matsui, & Touhara, 2016).
Research has shown that elephants can distinguish people from different tribes by odor and can
recognize up to 30 different family members.
There is a good argument to be made for the nose as a sexual organ. The most convincing
evidence comes from the study of pheromones, airborne chemicals released by an animal that
have physiological or behavioral effects on another animal of the same species. Most
pheromones are detected by the vomeronasal organ (VNO), a cluster of receptors also located
in the nasal cavity. The VNO is illustrated in Figure 7.7, although you will soon see that most
researchers believe that it is no longer functional in humans, the victim of evolution as our
ancestors developed color vision and came to rely on visual sexual signals (I. Rodriguez, 2004).
However, a VNO may not be entirely necessary, because some pheromones and pheromone-like
odors can be detected by the olfactory system when an animal’s VNO has been blocked or
eliminated surgically (Wysocki & Preti, 2004). The VNO’s receptors are produced by a different
family of genes, and the VNO and olfactory systems are separate neurally (P. J. Hines, 1997).
Not surprisingly, in animals the VNO’s pathway leads to the MPOA and the ventromedial
nucleus of the hypothalamus, as well as to the amygdala (Keverne, 1999).
Pheromones can be very powerful, as you know if your yard has ever been besieged by all the
male cats in the neighborhood when your female cat was “in heat.” The female gypsy moth can
attract males from as far as two miles away (Hopson, 1979). Pheromones provide cues for kin
recognition in animals, influence cycling of sexual receptivity in female mice, initiate aggression
in both males and females, and trigger maternal behavior in adults and suckling in infants
(Wysocki & Preti, 2004). In pigs, the boar exudes androstenone, which elicits sexual posturing
and receptivity in sows. In fact, pig farmers use androstenone as an aid in artificial insemination.
So, do pheromones play a role in human behavior? In spite of the eagerness with which the
media and fragrance industry have embraced the topic, the best answer appears to be “maybe . . .
maybe not.” We certainly don’t see pheromones controlling sexual behavior as powerfully as
they do in animals; in fact, the best candidate for pheromone control of human behavior is the
sucking and searching movements in infants in response to breast odors of a nursing woman
(Wyatt, 2016; Wysocki & Preti, 2004). Early interest in the possibility of human pheromones
was spurred by reports that women living together in dorms tended to have synchronized
menstrual periods and that this was caused by sweat-borne compounds that altered the frequency
of luteinizing hormone release (Preti, Cutler, Garcia, Huggins, & Lawley, 1986; Preti, Wysocki,
Barnhart, Sondheimer, & Leyden, 2003; K. Stern & McClintock, 1998). Later studies have failed
to demonstrate menstrual synchrony almost as often as they have succeeded, and the results have
been questioned on methodological grounds (Z. Yang & Schank, 2006).
Is there evidence for pheromones in human sexual behavior?
Several other studies have claimed evidence for an influence of pheromones, or at least body
odors, on human behavior. This includes amygdala activation from smelling the sweat of firsttime skydivers (Mujica-Parodi et al., 2009); increased intercourse opportunities when using
aftershave or perfume containing underarm extracts that enhanced the person’s sex-characteristic
body odor (Cutler, Friedman, & McCoy, 1998; McCoy & Pitino, 2002); higher alcohol
consumption and sociability in males after exposure to fertile female odors (Tan & Goldman,
2015); and men’s higher attractiveness ratings of the scent of women’s T-shirts when women
were ovulating (Kuukasjärvi et al., 2004).
Application: Of Love, Bonding, and Empathy
Source: Todd Ahern/Emory University.
Prairie voles are a rare exception among mammals; they mate for life, and if they lose a mate
they rarely take another partner. The bonding process (as reviewed in L. J. Young & Wang,
2004) begins with the release of dopamine in reward areas during mating. If dopamine activity is
blocked by a receptor antagonist, partner preference fails to develop. Sexual activity also releases
the neuropeptides oxytocin and vasopressin, which are likewise required for bonding to take
place. Either can facilitate bonding in males or females, but oxytocin is more effective with
females and vasopressin with males.
So does any of this apply to humans, who are also monogamous (more or less)? The most
apparent parallel involves oxytocin. Oxytocin not only facilitates bonding but also causes smooth
muscle contractions, such as those involved in orgasm and in milk ejection during breastfeeding.
Blood levels of oxytocin increase dramatically as males and females masturbate to orgasm (M.
R. Murphy, Checkley, Seckl, & Lightman, 1990). Oxytocin also contributes to social
recognition, which is necessary for developing mate preferences. Male mice without the oxytocin
gene fail to recognize females from one encounter to the next (J. N. Ferguson et al., 2000), and
human males are better at recognizing previously seen photos of women after receiving oxytocin
(Rimmele, Hediger, Heinrichs, & Klaver, 2009). Men given oxytocin also had more activity in
the nucleus accumbens while viewing photos of their partners, and they increased their ratings of
their partners’ attractiveness, but not of other women they knew (Scheele et al., 2013).
Oxytocin’s bonding effects are not limited to mates and sex partners. Mother-infant bonding is
correlated with oxytocin levels during pregnancy and following birth (Feldman, Weller,
Zagoory-Sharon, & Levine, 2007), and a gene for the oxytocin receptor is related to parenting
sensitivity (Bakermans-Kranenburg & van IJzendoorn, 2008). Oxytocin also apparently explains
empathetic behavior in prairie voles. Though we can’t speculate about what the rodents are
“feeling,” they respond to a cagemate’s earlier, unobserved stress with increased grooming, and
they match the cagemate’s fear response, anxiety-related behaviors, and corticosterone increase
(Burkett et al., 2016). Consoling behavior did n