Sept. 28, 2006— -- Most people think that the brains of baby boys and girls are blank slates at the beginning of life. But according to new scientific evidence, they're not.
Recent reports show that newborn males and females have very different brain circuitry, and hormones dramatically shape their future thoughts, feelings and behavior in the first years of life.
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"The day they're born their circuitry is already pretty much wired," says Dr. LouAnn Brizendine, a neuropsychiatrist and author of the new book "The Female Brain." "They're either formed as a male brain or a female brain."
Brizendine says that after eight weeks in utero all children's brains appear exactly the same: female. Female is nature's default setting. It is only after a surge of testosterone that boys' brains begin to look male.
Recent studies reveal that after birth girls are already better at reading faces and hearing human vocal tones. Incredibly, during the first two years of life a baby girl's ovaries will pump adult levels of estrogen. From six to nine months, a boy's testicles are flooded with adult levels of testosterone.
While the behavioral effect of all of this remains a mystery, conjectures have been made.
"The studies that were done with children around 12 months old where their moms went in a room with them and they were told not to touch an object," recounts Dr. Brizendine. "The boys would just go right for the object and touch it. The girls would hear their mom's voice, turn around, look at their mom's face, and stop. Boys don't hear the complete tones in the female voice."
"The reason little girls may play better together is brain wiring -- verbal ability at a younger age," she continued. "And they may just be able to negotiate the sharing better than boys. Of course, little boys and little girls act differently. Adult men and adult women act differently, too. But it has really been ignored until probably the middle '90s."
For years, brain studies focused primarily on male subjects. But in the '90s, the government began requiring that all its studies include women. At the same time, powerful new brain imaging technology and sophisticated animal studies provided scientists with tools to map human emotions and gender differences.
Due to this revolutionary technology, we now know that areas for emotional memory and communication are larger in the female brain, perhaps explaining why, on average, women remember fights that men insist never happened, and why women use 20,000 words a day, while men use only 7,000.
At puberty, there is again an explosion of hormones, according to Brizedine. As a female teen's brain emerges, hormones dramatically reorganize her brain circuitry, driving the way she thinks, feels, acts and even obsesses over her looks. Studies show that these surges of estrogen can trigger teen girls' need to become sexually desirable to boys.
The teenage male brain reorganizes too. Flooded with testosterone, many become absorbed in sexual fantasies and the need to masturbate.
"It's hard to believe that something as tiny as a little hormone could have such a robust behavioral effect for all of us," said Dr. Brizendine.
To further illustrate how a teen's changing brain chemistry often molds their behavior, Dr. Brizendine invited ABC's 20/20 to listen as she spoke to students at the Marin School outside San Francisco. Girls, she explained, mature faster than boys, and girls' brains are as much as two years ahead during puberty.
In fact, neuro-imaging shows that, early on, the typical teen girl has a stronger connection between the areas of the brain that control impulse -- the amygdala -- and judgment -- the prefrontal cortex.
It may not be until late adolescence or their early 20s that boys' brains catch up to their girl peers.
"To know that they're smarter than us by two years -- it's a gap, it really is," said John Bessolo, one of the students in Dr. Brizendine's high school group. "They are the superior beings of the brain."
But when it comes to sex, boys take the lead -- or least their brains do.
Dr. Brizendine explains that the male amygdala, which also controls sexual thought, is twice as large as that of females. Fueled by testosterone, it triggers the typical teenage male brain to think about sex every 52 seconds, compared to a few times a day for teen girls.
"When I hang out with my guy friends, it's really odd to think that every, like, whatever seconds, they're thinking about sex," said high school student Sara Johnson. "And that you can't stop them from thinking about that."
When it comes to emotions, Dr. Brizedine says girls have their own area that's twice as large as boys' -- the hippocampus, which is the seat of emotional memory. The female brain uses many centers in both hemispheres that activate in response to faces, voices and expressions. Men, however, use only one side of their brain.
Dr. Brizendine asked the teens to come up with questions they always wanted to ask the opposite sex. Teen girls, she explained, physically receive pleasure through the simple act of gossiping.
John posed the first question to the girls: "Why do you go to the bathroom in groups?"
"Girls are close to each other, and we like to do things together," responded Kelly Ericson, another member of the class. "It's more, 'I have to go pee. Oh, you have to go pee too? Let's go together.'"
Dr. Brizendine had a scientific explanation for this increased camaraderie in girls.
"Actually, there is a hormone that gets released when girls are, sort of more intimate with each other -- in terms of talking and fixing each other's hair and doing stuff. That's called oxytocin. It's a hormone that's released in the brain that's kind of a "feel good" hormone."
The hormone oxytocin gives a surge of pleasure, the same rush a drug addict gets from taking cocaine or heroin.When it was time for the girls to ask a question, Heidi, one of the teens in the class asked, "Why are guys able to like express everything sexually but not emotionally? Like, they can be like, 'Hey baby, what's up?' but, like, they can't, like, say, 'I'm sad.'"
Her classmate Chris Chun offered an explanation: "Guys aren't supposed to feel 'Oh, I feel sad cause my girl broke up with me.' And I think that's what sucks, is that we're expected to keep our feelings inside, and not have any."
A recent neuro-imaging study confirms that the average male, when tracking emotional expressions in the face of another, has fewer brain cells that light up than females do. Men are simply less equipped to read emotion.
Dr. Brizendine insists that despite the "hard wiring" of our brain chemistry, we do have free will. Clearly, not all women and men are the same. But for many the science is undeniable: powerful hormones and the complex circuitry of the brain do shape our behavior and, therefore, our destiny.
"It's biology, and it needs hands-on parenting,'" urged Dr. Brizendine. "It needs 'hands-on parenting.' That's why teens need a lot of wisdom and careful monitoring by adults in order to let the good connections grow strong and get rid of the bad connections."
And becoming a parent also sets in motion dramatic changes in the female mind.
During pregnancy, the biggest hormonal surge of a mother's life will send levels of estrogen and progesterone off the charts. It's something Sarah Cheyette, a pediatric neurologist and mother of three, knows all too well.
"If I'm crying hysterically about something during pregnancy, I feel, 'OK, it's the hormones and I'm not losing my mind entirely,'" said Cheyette.
"The progesterone level goes up -- way up," said Dr. Brizendine. "Thirty, 40 times what is normal… All of the surges and big changes that happen just wire you up for paying attention to that helpless little infant somehow."
During pregnancy, these powerful hormones literally hijack a mother's brain circuits. She first becomes sleepy, hungry and nauseous. Soon, the hormones oxytocin and prolactin intensely focus her maternal brain on the safety, and the needs of her child often to the detriment of everything else.
Scientists don't know why, but neuro-imaging studies show that pregnant women's brains actually shrink -- almost 8 percent -- during gestation. They return to their normal sizes six months after the mother gives birth, and then maternal aggression kicks in. Triggered by hormones, a mother's brain becomes a virtual GPS systems for tracking and protecting her young.
Studies have shown that, within about 48 hours or so, a mom can pick out the cry of her own baby above other babies. It could be that human evolution has been hard-wired in some ways to protect infants and propagate the species.
A similarly dramatic hormonal effect is experienced when mothers breastfeed. As she nurses, oxytocin, the feel-good hormone, marinates a mother's brain. Many women say they are awash in feelings of warmth and pleasure. According to Dr. Brizendine, "some women even say they almost have an orgasm while they're breast feeding -- that they can feel their uterus contracting."
Because of this, Dr. Brizendine suggests that it's no wonder that the mommy brain has little desire for sexual contact -- the baby is now her primary love interest, and the intense hormonal focus on her child leaves her satisfied, but exhausted.
"Having a baby definitely changes your sex life," said Cheyette. "The baby is taking up time and energy and brain space, and I think it's hard for a woman to get in the mood."
"The dad is there only in a supporting role now," adds Dr. Brizendine, "Whereas he's been used to being the main course, he's now like a side dish."
For many women, their child-centric behavior not only compromises their relationships, but also their jobs. Hormonally tethered to their child the interest of some mothers to return to work is often challenging.
"It's a struggle I live with everyday. Being at work, sometimes I feel guilty about being there. Sometimes being at home, I feel like I should be at work," said Cheyette. "And it's not just me, it's every mom that I know."
Balancing work, pregnancy and then family is a struggle for most women, and Dr. Brizendine hopes her book and research bring awareness about the difference between male and female brains.
Sarah Cheyette agrees: "For our children's sake, and for their children's sake, it would be best if people understood pregnant women, pregnancy and what being a mommy is about."