This Is Your Brain on Sex (13 page)

The average age of human menarche, or the start of menstruation, has decreased in the past hundred years. Some epidemiological studies have suggested that the abundance of fatty foods in our diet or the increased amounts of hormones we ingest may be at play. But just as much research has focused on the role of social environment. Sexual abuse, the presence of a nonrelated male figure such as a stepfather living in the home, and exposure to sexual stimuli have all been linked to early menarche. Later menstruation seems to happen in larger families or when a girl has a close relationship with her biological father, though the why’s and how’s of this phenomenon are unknown. There’s that context again, coming in and messing with our hormones. Taken together, the findings suggest that our environment can alter the way our hormones are expressed—and, by extension, our behaviors.

Though there is no specific hormonal event that triggers a girl’s first period, a rising level of estrogen has been noted. It is the same in the rhesus. The flood of estrogen that precedes menarche is a critical step toward the animal’s fertility. Social environment too is important. Social rank in the group is a factor that can predict how early the onset of puberty might be. Once again, environment and context
are influencing our hormones, not the other way around. “High-ranking females are more likely to go through puberty earlier than the low-ranking ones,” said Wallen. “One view of the amygdala is that it stamps social context into emotional memory. . . . So it’s possible that since puberty in females is socially mediated, in this case by rank, we questioned what the effect would be if you removed the amygdala.”

What happens to hormonal expression when you remove the ability to read social context? In the eight monkeys who received neonatal amygdalectomies, six had already gone through puberty, a full year earlier than females who have an earlier menarche. But more interesting to Stephens and Wallen was that the first monkey to undergo the procedure displayed normal ovulatory cycles but a complete and utter disinterest in soliciting sex, an important social behavior in rhesus females. “She didn’t show any interest in the males,” said Wallen. “The males would solicit her, but that was it.”

Unlike those brazen hussies who audaciously flashed their backsides to let Casanova know they were good to go, Opie, the first amygdalectomized monkey, never picked up the steps to the mating dance. It was unclear if she ever understood enough of the social context to realize there was a dance at all. She had normal cycles and the right hormone levels to have offspring, yet she never approached or solicited males for sex. The lack of amygdala meant that comprehending the social environment, especially when it came to mating rituals, was beyond her. It was a staggering revelation.

Stephen’s study is ongoing; the group is waiting to see if the other amygdalectomized monkeys will behave like Opie. But Wallen said that the preliminary results reinforced what he had been saying all along: we are not slaves to our hormones. Many factors, including social context, play a huge role. The hormones talk to us, definitely. But it’s not a “must-follow” kind of message. “My view of hormones is that they are really just suggestions,” Wallen told me. “From an evolutionary standpoint, all they’ve got to do is increase the probability that you’ll want to have sex at the right time for reproduction.”

This rings true with Micevych’s statement that, at the cellular level, hormones open the gates so we have more access to information. But they do not insist on a particular course of action,
nor do they take away our ability to choose how we behave. Ultimately, no matter what our hormone levels may be or how motivated we are to get busy, we still have the power to decide whether or not we will have sex, even when, as with Casanova, a sure thing is staring us straight in the face. As Wallen said, “We don’t have hormonal regulatory mechanisms. What humans have are simply motivational mechanisms. And motivations can be easily ignored or paid attention to, depending on context and environment.”

Hormones do a lot of influencing, that’s for sure, but we are far from their slaves.

Chapter 6

His and Her Brains

Bill Cosby, comedian and father
figure to the Generation X crowd, once joked, “Men and women belong to different species, and communications between them is still in its infancy.” It’s a great line because it feels like a basic axiom of intergender relations (and would appear so even if Dr. Huxtable had not been the one to utter it). Let’s face it, males and females seem to be fundamentally different.

How often have you responded to or dismissed some baffling behavior of the opposite sex with “He is
such
a guy” or “Must be a chick thing”? Even my five-year-old seems to understand that saying, “Because she’s a girl,” or “Because he’s a boy,” is an explanation unto itself. This is not misogyny on his part—he does not see it as a criticism—it is just simple rationalization. From an early age we all appear to implicitly understand that boys and girls, men and women, are different. Period. Some experts might even have you believe that men and women are so different that they are destined to always stand in opposition when it comes to both communication and behavior.

Since John Gray published his
New York Times
best-selling book
Men Are from Mars, Women Are from Venus,
we have tried to explain away all kinds of relationship troubles by lumping behaviors or traits into certain categories. Why don’t men call when they say they will? It’s a guy thing. Why do women lose interest in sex as they age? It must have something to do with their gender. Why do so many relationships fail? Men and women simply cannot relate without some kind of primer on translating each
other’s behavior. Many have argued that perhaps the biggest problem with sex and love (the heterosexual variety, anyway) is that men and women are too dissimilar to properly connect. Could the problems we face in love and sex come down to the differences between our genders?

In recent years countless scientists and authors have written thousands of pages discussing the distinctions between the brains of the two sexes—and by extension, the sex differences that are often seen in behavior. Some argue that although boys and girls may show natural variations in brain development and structure, nurture can always trump them.
¹
Others have promoted the idea of “neurosexism,” the suspicion that neuroscientific studies of sex differences in the brain, particularly neuroimaging studies, are simply a modern way to support the old notion that men are somehow intellectually superior to women.
²
We can leave both these concepts for others to support or refute. Instead I will explain what neuroscience has offered in terms of how these intergender brain differences develop and how they may apply to the study of sexual behavior and love.

“Eve Plus Androgen Equals Adam”

John Money was a leading sex researcher at Johns Hopkins University in the second half of the twentieth century. Much of his work involved the study of hermaphrodites, individuals born with both male and female or ambiguous genitals. Extending from that work, he also had quite a bit to say about gender’s influence on behavior and how observed differences between boys and girls may develop. In an unpublished manuscript originally intended for
Scientific American
titled “Pygmalion Updated,” he succinctly explained how hormones designate gender in the womb: “Eve plus androgen equals Adam.”
³

Catchy, no? But it does capture a rather complicated process in a simple way. Embryos pretty much all start out as female. They just don’t always stay that way. If the fetus in question gets a copy of the Y chromosome from dear old Dad, a flood of testosterone, an androgen, rocks the womb early in pregnancy, somewhere between six and twelve weeks. This increased level of testosterone guides the development of the penis, scrotum, and testicles. Without it,
the fetus would remain female and develop the corresponding reproductive tool kit.

But androgens do more than just determine what you are packing in your pants. Testosterone, estrogen, and progesterone are also critical to the brain’s development. They work in concert with a variety of other proteins and chemicals to help organize the brain into distinct regions and circuits. They are also responsible for sexual dimorphism, or systematic differences in form, in several areas of the brain.

The Sexually Dimorphic Brain

Historically when people talked about sex differences in the brain, the focus was on reproduction. It probably comes as no surprise that the hypothalamus, that little relay station to the pituitary gland implicated in all manner of sexual behaviors, is different in males and females. In rats a small cluster of cells in the hypothalamus called the sexually dimorphic nucleus has long been known to be significantly larger in male rats than in their female counterparts. What do I mean by “larger”? I mean that the actual volume of this area is larger relative to the overall volume of the brain it resides in. Work in animal models suggests that the bigger a brain area is, the more important it is. For example, rats rely more on their sense of smell than their eyesight to navigate the world; thus their olfactory brain areas are proportionately larger than their visual ones. Humans, who depend more on sight to get around, show the opposite effect when it comes to the relative sizes of these brain regions.

The human analogue to the rat’s sexually dimorphic nucleus is called the third interstitial nucleus of the anterior hypothalamus. This mouthful of a brain area has been linked to male sexual appetite as well as overall sexual behavior. Like the rat’s, it is bigger in men than in women. Given the variations observed in the outdoor plumbing, it is no surprise that you would see some sex differences in the brain regions associated with them. And for decades most researchers believed that was all there was to it.

New research, however, demonstrates that there are sex differences across the entire brain, and the brain areas affected by these gender-specific, in-utero hormone
baths influence a heck of a lot more than just reproduction. Brain regions involved in emotion, memory, learning, perception, executive function, and stress response also show some differences between the sexes. Jill Goldstein, a neuroscientist at Harvard Medical School, used neuroimaging to look at sex differences across the whole brain. She and her colleagues discovered that parts of the frontal cortex are bigger in women than in men, as well as several brain regions in the limbic cortex, the emotional response area of the brain. In men, the parietal cortex and amygdala, on average, are larger. These areas are involved in spatial perception and navigation and emotional arousal and salience, respectively. Even more interesting is that all of these areas show a very large number of sex hormone receptors during embryonic development. Those hormones in the womb are making some important changes. And when puberty brings another hormonal onslaught later in life, these circuits are activated and ready for business.
⁴

As I said, the long-held assumption is that the bigger the area, the more important it is to the organism in question. As researchers looked deeper, they found that these larger areas also demonstrated greater densities of neurons as well as more dendritic growth, which is an indicator of neural reach; the more dendrites on a cell, the more synapses it can form. Did these variations designate a difference in cognitive processing between the two sexes? Anecdotal evidence certainly suggested so. But it was not until neuroscientists started using fMRI during cognitive tasks that they knew for certain.

Larry Cahill is a researcher at the University of California, Irvine, who studies emotional memory. He often uses neuroimaging studies in his research. Several years ago he noticed an interesting quirk between male and female study participants during an fMRI study. He already knew that the amygdala was usually larger in men and therefore suspected that it might show more activation during certain kinds of emotional tasks. But he was surprised to find that men and women showed varying baseline activation in the amygdala. That is, when the study participants were only relaxing inside the magnet (as much as one can relax inside an fMRI), their amygdalas were lighting up in different ways.

This was a big deal for cognition and for the study of love-related behaviors. Many of the neuroimaging
studies found in the scientific literature, including some that look at love and sexual behavior, take blood flow measurements in only one gender. Any conclusions drawn from the results have to be reconsidered when you realize that male and female brains are just a little bit dissimilar.
⁵
In light of Cahill’s work, it would be all too easy to throw the baby out with the bathwater and turn your nose up at any neuroimaging study that scans only one gender. Not to mention that the idea of “his” and “her” brains is quite the political topic. In this postfeminist world, all too often the notion of “different,” as demonstrated in scientific study, morphs into “better” or “smarter.” But researchers like Cahill and Goldstein are careful to point out that just because male and female brains might develop from a slightly different blueprint, their behavioral output may not differ all that much. Take stress, for example. “Male and female brains work slightly differently under stress, even though this may lead to similar levels of performance,” said Goldstein. “I often use the example of a Mac and a PC computer. They have been built differently, but they can accomplish the same kinds of tasks at a similar performance level.”