Welcome to Your Brain (25 page)

brains of these two closely related species, scientists have learned a lot about the neural basis of

pair-bonding. To measure bonding in the laboratory, scientists allow one vole to wander freely

through a container with three rooms connected by tubes. The vole that’s being tested is placed in an

empty room, which is connected by two passageways to a room containing the vole’s mate and

another room containing a stranger. The more time the vole spends in the room with its mate, the more

bonded it is. Unsurprisingly, the strongest stimulus for formation of a pair-bond is having sex with the

partner, but some prairie voles become bonded simply by living together.

Two neuromodulators, oxytocin and arginine vasopressin (AVP), control the formation and

expression of pair-bonds in voles. Both these neurotransmitters are important for social recognition in

rodents. Oxytocin is released in many mammals during vaginal or cervical stimulation, including

childbirth and mating. Oxytocin is important for mother-infant bonding in many species; it seems to be

more important for pair-bonding in female voles than in males. On the other hand, AVP is important

for a variety of male behaviors—including aggression, scent marking, and courtship—and seems to

be the main pair-bonding hormone in male voles. In a pinch, though, either peptide can induce pair-

bonding in voles of either sex. Infusion of either neurotransmitter into the brain causes pair-bonding to

occur after a short exposure to the partner, even if the pair has not had sex.

The monogamous prairie voles have more receptors for both these neurotransmitters than do the

promiscuous meadow voles in certain key brain areas. The two regions that seem to be important for

partner preference are in the core of the brain: the nucleus accumbens, which has a high density of

oxytocin receptors, and the ventral pallidum, which has a high density of AVP receptors. Locally

blocking either set of receptors prevents pair-bonding, as does blocking oxytocin receptors in the

prefrontal cortex or blocking AVP receptors in the lateral septum of males. All these areas are

considered to be part of the brain’s reward system (see
Chapter 18)
. Release of the neurotransmitter

dopamine within this circuit is critical for the response to natural rewards, like food or sex, and to

addictive drugs.

Indeed, love may be the original addiction. Why does the brain have pathways devoted to making

people crave white powders that never occur naturally? Perhaps because the brain regions that are

important for drug addiction are also the neural circuits responsible for responses to natural rewards,

including love. If the ability to become addicted helps animals bond to their mates, perhaps that’s

why these neural pathways are useful for the survival of the species—and why they persist in spite of

the harm that addiction can cause.

Pair-bonds seem to form by conditioned learning, in which the partner’s smell (in the case of

rodents, at least) becomes associated with the rewarding feelings of sex. This is no different in

principle from teaching your dog to sit by associating this behavior with giving him something to eat

—both eating and sex increase the release of dopamine in the nucleus accumbens. Blocking a

particular subtype of dopamine receptor prevents the development of mating-induced partner

preference, while activating dopamine receptors induces partner preference without mating. After

two weeks of bonding with a female, the male prairie vole develops an increased density of another

subtype of dopamine receptor that reduces pair-bond formation, presumably to make it harder for him

to form a new bond with another female that might interfere with his first pair-bond.

Did you know? Studying flirtation

Both women and men tend to think of men as the initiators of sexual relationships, but

psychologists who study human courtship behavior have disproved that stereotype.

Observational study in singles bars (nice work if you can get it) shows that men rarely

approach a woman until she has given them a nonverbal signal that it’s okay to proceed.

Solicitation behaviors—defined as any body movement that caused a male to move closer

within fifteen seconds—were mostly unsurprising, and included glancing, primping,

smiling, laughing, nodding, requesting aid, and touching the other person. Less attractive

women with high levels of solicitation behavior were more likely to be approached by men

than more attractive women with low levels of solicitation behavior. In fact, researchers

were able to predict whether a woman in a bar would be asked to dance within the next

twenty minutes with 90 percent accuracy just by recording how often she did things like

glance around the room, smile at a man, or smooth her hair in a ten-minute period.

The most convincing evidence that these neural systems account for pair-bond formation is that

scientists have succeeded in converting the promiscuous meadow vole to monogamy by

experimentally inducing expression of the AVP receptor in its ventral pallidum. This amazing result

shows that a complex behavior like pair-bonding can be turned on or off by a single gene in a single

brain area, although, of course, other genes in other brain areas are required for the behavior’s full

expression once the switch is flipped.

Mothers’ attachment to their children may involve some of the same neural circuits as bonding

with a mate. As we’ve already mentioned, oxytocin is necessary for mother-infant bonding. When

rodents who have never had pups are given oxytocin, these inexperienced females will approach pups

and try to care for them, instead of being aggressive toward them as a female nonmother normally

would be. Blocking oxytocin receptors during labor and delivery prevents rodent mothers from

bonding with their pups. Damaging the ventral tegmental area or the nucleus accumbens, both of

which are associated with rewards in female rodents, also impairs their ability to care for their pups.

But enough about prairie voles, cute as they may be. You’re probably wondering if this is how

people fall in love. We don’t know for sure, but there’s some evidence that the idea is plausible.

Oxytocin levels increase during orgasm in women, and AVP concentrations increase during sexual

arousal in men. In addition, functional imaging experiments suggest that romantic love (in both sexes)

and male orgasm activate similar reward areas of the brain, the ones that contain receptors for

oxytocin and AVP. People who are intensely in love show activity in the ventral tegmental area and

the caudate, while people in longer-term relationships (about a year) also show activation of other

regions, including the ventral pallidum (the site of prairie vole bonding), when they look at a picture

of their lover. These findings suggest that romantic love in humans may involve oxytocin, AVP, and

the brain’s reward circuitry—all of which are important for pair-bonding in voles.

If you’ve taken stupid risks when you were in love—and later wondered how you could have

trusted that loser—you might be interested to know that oxytocin also seems to increase people’s trust

during social interactions, even with strangers. Subjects in one experiment were asked to play a game

in which an investor could make money by taking the risk of giving some money to a trustee, who

would then get the investor’s money plus a bonus and could choose how much of it to give back to the

investor. If the trustee is trustworthy, both players benefit from the investor’s decision; otherwise only

the trustee benefits. Investors who were given oxytocin (via a nasal spray) were about twice as likely

to give money to the trustee as those who were not given the drug. This effect was only seen when the

trustee was a real person, not when a computer randomly decided how much money the investor

would get, so oxytocin seems to be involved specifically in social interactions, not in risk taking

more generally. These results suggest that you might want to avoid making important financial

decisions while under the influence of mind-altering substances, like those released during orgasm.

Did you know? Imaging orgasm

You’d never get this approved by a university in the U.S.: a group of Dutch scientists

has been studying human brain activity during orgasm by using positron emission

tomographic (PET) scanning. Of course, the brain’s reward system is activated during

orgasm in both sexes. In addition, women showed reduced activity in an area of the frontal

cortex, which might relate to a reduction of inhibition. Men showed reduced activity in the

amygdala, suggesting a relaxation of their vigilance during orgasm. Both sexes showed

increased activity in the cerebellum, which has recently been implicated in emotional

arousal—and sensory surprise.

Myth: Men learn to be gay

Research suggests that many homosexual people are born that way—though the

evidence is much more clear for gay men than for lesbians. Factors that affect the

development of male fetuses also influence adult sexual orientation. Some of these factors

are probably genetic, as homosexuality is significantly heritable in human twin studies,

while others are environmental influences from the mother during pregnancy. This research

doesn’t prove that environmental influences after birth are irrelevant, but it does suggest

that it is possible to develop a homosexual orientation without learning.

Children with disorders of sexual development provide an opportunity to test this idea

because they often have known abnormalities in prenatal hormone exposure. For example,

in a syndrome called congenital adrenal hyperplasia, a genetic defect causes female (XX)

babies to produce a male steroid hormone that masculinizes their brains and sometimes

their genitalia. Even when the hormone defect is corrected with medication after birth,

these females are much more likely than normal women to have adult sexual fantasies and

experiences involving other women. Women whose mothers took the drug DES, another

masculinizing agent, once thought to prevent miscarriages, also are more likely to be

attracted to other women, though their genital sex is normal. At the opposite extreme, males

with androgen insensitivity have a genetic defect in the receptor for the male hormone

testosterone. Because their bodies and brains are not responsive to male hormones, these

genetic males (XY) are born with female genitalia and typically raised as girls. Nearly all

of them report being attracted to males in adulthood, suggesting that sexual attraction to

women requires prenatal brain masculinization by hormones.

If homosexuality is due to early hormones, then we’d expect gay men to look more like

women in brain regions that differ between the sexes. In humans, the strongest sex

difference occurs in a region with the tongue-twisting name of the third interstitial nucleus

of the hypothalamus, which on average is more than twice as large in men as in women.

Two studies have reported that this region in gay men is about the same size as in women.

As far as we know, no one has studied this region in lesbian brains.

For men without a medical disorder, the strongest known predictor of homosexuality is

having an older brother. This effect has been found in more than a dozen studies. Each older

brother increases the odds that a later-born male will be gay by a whopping 33 percent.

That is, if gay men are 2.5 percent of the male population (which is roughly correct), a boy

with one older brother would have a 3.3 percent chance of growing up to be gay, and a boy

with two older brothers would have a 4.2 percent chance. According to these statistics,

roughly 15 percent of gay men owe their sexual orientation to their older brothers. In

contrast, there appears to be no birth order effect on homosexuality in women.

No one is quite sure how having older brothers affects sexual orientation. It isn’t

because of the mother’s age, as this doesn’t happen in firstborn male children of older

mothers, and it doesn’t matter if the older brother is in the house when the younger brother

is growing up. The effect probably occurs before birth; homosexual males with older

brothers weigh less at birth than heterosexual males with the same number of older

brothers. Researchers’ best guess at the moment is that the immune system of women who

are carrying a male fetus may make antibodies against some factor that males produce,

which would then act to suppress that factor in subsequent male babies. One candidate is

the Y-linked minor histocompatibility antigen, though the only evidence in its favor so far is

from rats; immunizing rat mothers against this protein reduces the likelihood that their male

offspring will mate with females and reproduce.

Taken together, all this research suggests that brain development during pregnancy has a

significant effect on adult sexual orientation. We can’t deny that the expression of people’s

sexuality also is richly influenced by their life history, but it looks as though the basic plan

is laid out early in life.

Life decisions aside, the most dramatic sex differences in the entire brain are found in the parts

that control what you do in bed. Here we’re not talking about sex differences in cognition, which are