know what they're doing: Eaton 2008 found that in the United States, 72 percent of all deaths among persons age 10-24 result from four causes: motor-vehicle crashes, other unintentional injuries, homicide, and suicide. The 2007 national Youth Risk Behavior Survey (YRBS) indicated that many high-school students engaged in behaviors that increased their likelihood of death from these four causes.
biologically ready to handle independence: Doremus-Fitzwater 2010 found that biological changes in the brain's motivational and increase teens' peer-directed taking, novelty seeking, and drug and alcohol use relative to adults. For more on how teens' sleep deprivation influences risky peer interactions, see Dahl 2008. reward-related regions
social interactions, risk
prefrontal cortex (PFC)--is like a brake: Steinberg 2004 and 2007.
boys until their early twenties: Giedd 1996 and 2009. mother's body, but also by her smell : Savic 2001 and Weisfeld 2003 found that the odor of genetically related family members is not romantically attractive. For more on MHC genes and odorous attraction, see Garver-Apgar 2006, Wedekind 1995, and Yamazaki 2007.
to have fleeting sexual fantasies : Campbell 2005 studied schoolboys age 12-18. The study found spontaneous nocturnal emission, secondary sexual characteristics, and salivary testosterone correlated with age at first sexual fantasies, noncoital sexual behavior, and coitus. They found first erection at an average age of 10.75 years, first sexual fantasy at 12.66 years, first spontaneous nocturnal emission at 13.02 years, and adult levels of blood testosterone at 17.2 years. Carlier 1985 found that boys' testicle size correlated best with first ejaculation.
when boys begin frequent masturbation : Korkmaz 2008 found that over 90 percent of 16-year-old boys masturbated and 98 percent reported that they liked it and thought it was natural. However, some felt guilt, fear of harming one's body, or shame. Some boys felt sexually inadequate when they compared themselves with their peers, feeling they were not as attractive to girls. For more on sexual behavior in teen boys, see Giles 2006, Auslander 2005, and Browning 2000.
to three times a day: Tanagho 2000. less than one time per day : Korkmaz 2008. Gerressu 2008a found that 95 percent of men and 71 percent of women masturbated. And one consistency across all studies was the large gender difference in the prevalence and frequency of masturbation--both being much greater in males. For more on sexual frequency in males and females, see Kontula 2002, Hyde 2005, Dekker 2002, Pinkerton 2002, Langstrom 2006, Giles 2006, and Laumann 1999b.
at the first opportunity: Tanagho 2000. finally came to "do it" : Adolescents are faced with many developmental tasks related to sexuality, such as forming romantic relationships and developing their sexual identities. Almost half will engage in vaginal sexual intercourse by the end of high school. For more on puberty and sexual development, see Eaton 2008 and Auslander 2005.
THREE: THE MATING BRAIN: LOVE AND LUST
The Mating Brain: Love and Lust : For a review on similarities and differences in the mate preferences and choices of women and men, see Geary 2004 and Young 2008.
lit up like a slot machine : For more on brain circuits for male courtship behaviors, see Pfaff 2002, Fernandez-Guasti 2000, Wu 2009, Maner 2007b, and Manoli 2006.
of his ancient mating brain : Voraceck 2006 found that the ultimate goal of female physical attractiveness is to elicit male sexual arousal. They found that males focused more on waist-to-hip ratio in women they saw moving and more on bust size in women who were stationary. In humans, the nucleus of the preoptic area of the hypothalamus is two to two and a half times larger in the male brain compared with the female. And Welling 2008 found that changes in testosterone levels contribute to the strength of men's attraction to femininity in women.
in men across all cultures: Singh 2002. mate-detection circuit was visual : Tsujimura 2009 found that in the non-intercourse video clip, gaze time for the face and body of the actress was significantly longer among men than among women.
mating brain read Nicole right : Amador 2005 found that both sexes place high value on traits like dependable character, emotional stability/maturity, and pleasing disposition, as well as mutual attraction and love. Women in the study placed industrious character, and good financial prospects. Men cared more about the woman's fitness and good health, good cooking/ housekeeping skills, and good looks.
higher emphasis on ambitious/ similar educational background,
cute and looked harmless enough : Maner 2008 and Shoup 2008. For more on mating judgments of men versus women, see Gangestad 1993 and 2000.
cues as Ryan and Nicole: Eibl-Eibesfeldt 1972. not out of his league : Bateson 2005 and Alpern 2005 found that both men and women become less choosy over time, as the highest-ranking and most fit and attractive individuals pair off first. This means that for a male to be chosen by her as the best, all he has to do is outshine the other males he is being compared with on the specific characteristics she is focusing on.
"I'm here to watch you": O'Hair 1987 and Farrow 2003.
wrong with meeting those expectations: O'Hair 1987 and Haselton 2005. with the highest-pitched voices : Apicella 2009. Sokhi 2005 found that female and male voices activate different regions of the male brain. Hughes 2008 and Pipitone 2008 found that a woman's voice attractiveness varies across the menstrual cycle.
go weak in the knees : And Roney 2008 found that women are more attracted to men's masculine characteristics, like jutting jaw and large muscles, during the ovulatory phase of the menstrual cycle.
potentially a good genetic match : Savic 2001b found that men are most attracted to the scent of women who are genetically Lundstrom hormonal birth control do not make the same pheromones or have the ovulatory-phase rise in testosterone derivatives like androstenedione that stimulate the sweat glands to produce those feminine "come-hither" pheromones.
different from them. And according to 2006, women who are on the "pill" or
give birth to sickly offspring: Alvarez 2009. best to each other: Wedekind 1995. Yamazaki 2007 found that dissimilar gene-type MHC emits special body odors that underlie mate choice and familial recognition, which helps make sure that inbreeding with parents and siblings does not occur. For more on brain responses to pheromones, see Hummer 2010, Mujica-Parodi 2009, and Prehn-Kristensen 2009.
and not even known why : Li 2007 says that people's odors can increase or decrease their likeability rating. Furthermore, according to Berglund 2006 and Sergeant 2007, gay males' and gay females' brains respond positively to same-sex pheromones. They dislike those odors of the opposite sex. For more on mating and pheromones, see Savic 2001a and 2009 and Zaviacic 2009.
about hygiene; it's about genes: Weisfeld 2003 and Olsson 2006. Havlicek 2009b found that olfactory and visual channels may work in a complementary way in mate attraction to achieve an optimal level of genetic variability.
could talk to her about : Keverne 2007 suggests that a male's ability to find a fertile mate requires some serious strategic maneuvering and that for human males, these reproductive strategies are complex and embedded in the social structure and hierarchies of society. So success in human males usually depends more on intelligent behavior than on hormones or odors.
tension between them was palpable : Roney 2007 found that a man's testosterone goes up just from talking to a woman.
secretly sent to their brains : Gallup 2008 and Hughes 2007 found that kissing is a mate-assessment device. Wyart 2007 found that testosterone and its metabolites are found in male saliva, semen, and sweat--and they smell, and perhaps taste, delicious to a woman when she is ovulating. While females find the masculine odor attractive, heterosexual males dislike it. For more on pheromones and mating, see Bensafi 2003 and Walter 2008.
center in a woman's brain : Muir 2008 suggests that a man's excretions could be absorbed by a woman during kissing, touching, and skin-to-skin body contact and thereby affect her brain.
Nicole was being so cautious : Hill 2002 found women more cautious about moving toward sex too soon and m
en needing less of a sense of emotional investment in the relationship before having sex.
up to three times longer: Buss 1993. anything, least of all sex : Roese 2006 found that women regret having sex early in the dating relationship more than men do.
offspring he's likely to have: Buss 1993 found that men's wanting sex with many women has likely been evolutionarily selected for in males.
of education or financial independence: Buss 2005 and Jensen-Campbell 1995. and was willing to invest: Griskevicius 2007 found specific mating goals increased men's willingness to spend money on conspicuous luxuries for women. They say that romantic motives produce highly strategic and sex-specific self-presentations. And Klapwijk 2009 found that generosity serves the important purpose of communicating "trust."
males who bring them meat: Gomes 2009. men, sex often comes first: For more on gender differences in love, commitment, and sex, see Roese 2006, Sprecher 2002, Keverne 2007, Loving 2009, McCall 2007, Geary 2000, and Buss 1993.
side-blotched lizard (Uta stansburiana): Bleay 2007. "How can I pick a blue-throat?" : Humans have a mating system scientists refer to as mild polygyny--multiple partners combined with a variable commitment to male parenting. Andrews 2008 found a sex difference in detecting infidelity--men are better at it. Atkins 2001a found that 20 percent to 25 percent of the married American population had had episodes of infidelity. Kontula 1994 found that in Finland, 52 percent of the men and 29 percent of the women reported episodes of infidelity in their lifetimes. They found that men reported being less emotionally involved than women with their infidelity partners, whereas the women seemed to connect both emotionally and sexually.
aggressively reject all other females : Gobrogge 2007 found that passionate mating changes the male brain biologically forever and that it leads pair-bonded male prairie voles to reject new fertile females. They found that it is an interaction between dopamine and vasopressin that results in pair bonding in the male brain's hypothalamus and nucleus accumbens, NAc. For more on hormones, genes, and pair bonding, see Winslow 1993, Carter 1998, Liu 2001 and 2003, Lim 2004c, and Young 2009a. (In the female brain, it is oxytocin and dopamine that interact to produce pair-bond formation.)
preference for this one female : For more on sex and partner preference in mammals, see Carter 1998 and Young 2008.
bond with their sexual partners : Liu 2001 found that when experimenters gave a chemical to block the vasopressin receptor, it blocked the intercourse-induced pair bonding in the males.
in their brains couldn't merge : For more on hormones in the brain, sex, and pair bonding, see Young 2008 and 2009b, Carter 1998, Becker 2008b, Wang Z. 2004, and Pfaff 2002.
vole, he, too, became monogamous: Lim 2004c experimentally induced pair-bond formation in the promiscuous vole by inserting the vasopressin gene from the monogamous prairie vole into the promiscuous vole.
this vasopressin receptor gene too : For more on the vasopressin receptor gene in humans, see Aragona 2009, Adkins-Regan 2009, and Walum 2008.
to one woman for life : Walum 2008 found an association between one of the human vasopressin receptor genes and traits reflecting pair-bonding behavior in men. They showed that the vasopressin genotype of men also affects marital quality as perceived by their wives.
mating strategy for short-term partners: Haselton 2005.
to have sex with them: Haselton 2005.
and business and social connections: Reviewed in Shackelford 2005d and Buss 2005b.
and brain closer to Frank: For more on female brain, oxytocin, and pair bonding, see Liu 2003. the more he squirmed : Loving 2009 found an increased stress reaction in men when discussing commitment or marriage.
electrical strain while they lied: O'Hair 1987. couldn't get enough of her : Gillath 2008a found that sex increases the desire to share personal information, fosters intimacy-related thoughts, and promotes a willingness to sacrifice for one's partner. Klusmann 2002 found that although sexual activity and sexual satisfaction decline in women and men as the duration of the partnership increases, sexual desire declines only in women, not in men. (And the desire for tenderness declines in men and rises in women.) They conclude that a stable pair bonding does not require high levels of sexual desire for women, after an initial phase of infatuation has passed. But for men the opposite is true. They found that male sexual desire should stay at a high level because it was selected for in evolutionary history as a precaution against the risk of sperm competition.
necessary part of getting there: For more on the male brain, pair-bond formation, and intercourse, see Liu 2001. getting a primitive biological craving : For more on the specific brain areas where dopamine exerts its effects on pair bonding, pleasure, reward, and motivation, see Curtis 2006.
neurotransmitter for motivation and reward : Aragona 2009 found that dopamine transmission mediates the formation and maintenance of monogamous pair bonds. For more on motivation and reward in pair-bond formation, see Kruger 1998, Exton 2001a, and Young
anticipation of pleasure and reward : Knutson 2008 found that the nucleus accumbens (NAc) activation increases during anticipation of pleasure and deactivates during anticipation of loss in a relationship.
mixed with estrogen and oxytocin: Both males and females have oxytocin, vasopressin, testosterone, and estrogen, but the ratios are sex-specific and controlled by genes, proteins, and enzymes like aromatase. For more on the male brain, estrogen, and aromatase, see Wu 2009. For more on pair-bond formation in males and females, see Liu 2003, Bocklandt 2007, Becker 2008a, and Carter 2008.
head over heels in love: For more on the brain and intense romantic love, see Aron 2005 and Fisher 2005 and 2006.
their bodies and brains became: Gonzaga 2006.
moments daydreaming about their lovers: Fisher 2004. they focused only on Nicole : Fisher 2006 found that when the in-love subjects looked at their beloveds, men also showed positive activity in a brain region associated with erection hardness. This means that the male love-response directly links romantic associated with sexual describes specific brain circuits for unconditional love. passion with a brain region
arousal. Beauregard 2009 to hold on to her : Buss 2002 says that the male must be fending off potential mate poachers and preventing his mate from defecting to hold on to the female. He found that mate-guarding adaptations evolved to avoid suffering negative reproductive costs, ranging from genetic cuckoldry to reputational damage to the permanent loss of a mate, and that male mate-guarding behaviors can range from vigilance to violence.
happen several times a day : For more on the male brain, lust, and visual sexual attraction circuits, see Fisher 2002, 2005, and 2006.
imagined Frank hitting on her : Rilling 2004 reviews sexual jealousy in males. Little 2007 and Burriss 2006 found that men sense a preference shift in their female partners toward more masculine men at ovulation. For more on male sexual coercion, see Starratt 2008 and 2007.
a tactic called mate-poaching : Schmitt 2004 found that the patterns for men in mate-poaching are similar across fifty-three nations. And they found that women poach too. Parker 2009 found that when a man was described as "unattached," 59 percent of the single women were interested in pursuing him, but when that same man was described as "being in a committed relationship," over 90 percent of the women expressed interest in the guy.
intensify our feelings of love : For more on rejection intensifying emotional commitment, see Baumeister 2001, Eisenberger 2004, Macdonald 2005, and Fisher
and possessive mating instincts wild : For more on mating instincts and hormones, see Carter 2007 and 2008, Becker 2009, and Pfaff 2002.
FOUR: THE BRAIN BELOW THE BELT
average of one or two : For more on men wanting a greater number of short-term sexual partners, see Schmitt 2001.
interest in one-night stands: For more on one-night stands, see Schmitt 2001, Laumann 1999b, and Mulhall 2008a. For more on men's satisfaction with their sex life, see Colson 2006, who found that almost 70 percent of men re
ported that they wished to change some things about their sexual life.
men's testosterone levels to go up: Van der Meij 2008. as sexually hot--or not : Ortigue 2008 found that the male brain's decision about desirability of sexual stimuli occurs within the first 200 milliseconds after seeing a woman. This means it occurs before conscious processing.
way their penis is shaped: Sanchez 2007. happy with their partner's size : Lever 2006 surveyed 52,031 men and women and found that many men wished they had a larger penis. And only 2 out of 1,000 men wished their penis were smaller. Dillon 2008 found that penile size is a considerable concern for many men from teens to old age. Wessells 1996 found that neither a man's age nor the size of his flaccid penis accurately predicted erectile length. But stretched penis length most closely correlated with erect penis length. For more on penis size, see Francken 2002.
is their most important feature : Francken 2002 found that a great many men believe that the size of the penis is directly proportional to its sexual power.
larger than it needs to be : Diamond 1997 notes that since the penis only has to be able to fit into a woman's vagina, men with penises that are too large may not be able to sire as many offspring, thus making larger penises undesirable.
from 5.5 to 6.2 inches : Wylie 2007 found that the average erect penis is 5.5 to 6.2 inches long and that an average-sized man is likely to be troubled by concerns that his penis is not large enough to satisfy his partner or himself and to be ashamed to have others view his penis, especially in the flaccid state.
to their females, it's supersize : Diamond 1997 describes that, compared with other mammals, the human penis is larger than necessary.