Gender and the brain
New evidence shows how hormones wire the minds of men and women to
see the world differently
By Ronald Kotulak
Tribune science reporter
Published April 30, 2006
Scientists are still a long way from figuring out what women and men
really want, but they are getting a lot closer to understanding what
makes their brains so different.
That women and men think differently has little to do with whether
they are handed dolls or trucks to play with as infants. After all,
when infant monkeys are given a choice of human toys, females prefer
dolls and males go after cars and trucks.
The differences, researchers are beginning to discover, appear to
have a lot more to do with how powerful hormones wire the female and
male brain during early development and later in life.
Among the newest findings: A previously unknown hormone appears to
launch puberty's sexual and mental transformation; growth hormone is
made in the brain's memory center at rates up to twice as high in
females as in males; and the brain's hot button for emotions, the
amygdala, is wired to different parts of the brain in women and men.
Scientists hope the findings may help explain such mysteries as why
females are often more verbal, more socially empathetic, more
nurturing and more susceptible to depression, while males tend to be
more aggressive, more outdoorsy, more focused on things than people
and more vulnerable to alcohol and drug addiction.
"Males and females look different, we act different, so of course
our brains are different," said Rutgers University psychologist
Tracey Shors, who is studying the effects of growth hormone on the
brain. "Sex hormones along with stress and growth hormones change
the brain's anatomy, and in that way you change behavior, your
ability to think and learn."
Sex differences begin with the X and Y sex chromosomes a person is
born with. But scientists now believe that whether the brain and
nervous system are wired as female or male depends a lot on the
early influence of estrogen, the so-called female hormone, or
testosterone, the male hormone.
The brain's sexual identity is first established when those hormones
are briefly released before and shortly after birth, which may
influence a child's preference for dolls or trucks.
"There's a peak of testosterone in males at birth that's very
important for future sexual behavior," said Dr. Sophie Messager of
Paradigm Therapeutics in Cambridge, England. "If you block that, the
male rats behave like females for the rest of their life."
The sex hormones then lie dormant until they get turned on again in
puberty to make the body ready for reproduction.
That is where a recently discovered hormone called kisspeptin comes
in. Created in the brain, it unleashes a cascade of hormones that
race down to the gonads--ovaries in females and testes in males.
There they stimulate the production of estrogen or testosterone,
starting the physical transformations of puberty. Messager proved in
animals that blocking kisspeptin prevented those changes from
But there is another target for this activity: the brain. The
hormonal downrush kicked off by kisspeptin comes full circle when
estrogen and testosterone travel back to the brain, imprinting
neural circuits with female and male characteristics, Messager said.
Animal studies show that genetic females will behave like males if
their estrogen is blocked and replaced by testosterone. Genetic
males, in turn, act like females if their testosterone is knocked
Until kisspeptin was discovered, scientists had generally accepted
the idea that sex differences were centered in the hypothalamus, a
small organ on the underside of the brain. It was thought that the
hypothalamus originated the flow of hormones that start puberty,
determine male and female physical characteristics and orchestrate
"The bias of mainstream neuroscience for the last 25 years has been,
`OK, sure there's some sex differences way down deep in the brain in
this little structure called the hypothalamus, but otherwise the
brains of men and women were pretty much the same,'" said Larry
Cahill, a neurobiologist at the University of California, Irvine.
"That was wrong, as wrong as could be," said Cahill, who is using
imaging technology to show how male and female brains are wired for
emotions. "Sex matters a lot in how the brain works and we
neuroscientists have to change our tune."
One example lies in the amygdala, the organ that interprets the
emotional content of an experience, affecting what people remember.
Located deep in the brain on both sides, the amygdala amplifies
memories that are pleasant or frightening. It tells the hippocampus,
where memories are put together to be stored, which memories need to
be most tightly locked in place. It will never let you forget what
you were doing when you won the lottery or where you were on Sept.
Cahill and his colleagues found that the amygdala works differently
in men and women, which may help explain why women are more likely
to develop mood disorders such as depression and men are more prone
to alcoholism and drug abuse.
In one experiment, Cahill showed that when men and women watched the
same emotional movie, the right side of the amygdala was more active
in men, and the left amygdala was more active in women. "They're
using very different brain processes to create enhanced memories,"
The right amygdala is more in tune to the outside environment,
communicating with the visual cortex, which controls vision, and the
striatum, which coordinates motor actions. These processes are
thought to be key to spatial orientation--knowing how to negotiate
your surroundings, as in hunting.
The left amygdala is concentrated more on the inner environment of
the body, connecting with the insular cortex, which produces
emotionally relevant content from sensory experiences, and the
hypothalamus' regulation of the body's metabolic and autonomic
activities. Scientists speculate that this is important for the
female capacity for nurturing.
A second study by Cahill involved the beta blocker propranolol, a
drug used to treat high blood pressure that also has been found to
greatly reduce the activity of the amygdala. Because it subdues
emotional arousal propranolol is being studied as a way to reduce
the impact of post-traumatic stress disorder.
In Cahill's experiment, normal subjects were given propranolol
before seeing an emotionally disturbing movie about a boy run over
by a car. Cahill found that women on the drug were able to remember
the central idea of the story, such as that the boy was with his
mother, but fewer of the details. Men, on the other hand, remembered
more details, like the soccer ball the boy was holding, but less of
the essence of the story.
"The drug impaired memory for the details of the emotional story in
women but not men, and it impaired memory for the gist of the story
in men but not women," Cahill said.
One possible explanation for why women tend to be less aggressive
than men is that they may be better able to filter out overly
arousing feelings. The front part of the brain, which controls
emotions, is bigger in women than in men when compared with the size
of the amygdala, where experiences get their emotional charge.
That difference may be why women are less prone than men to fly off
the handle, Cahill said.
Scientists also have made new discoveries about growth hormone,
whose chief job was thought to be to build the body. But researchers
have found the hormone is produced not only in the pituitary gland
but also in the brain, in the hippocampus.
That suggests the hormone plays a previously unsuspected role in
learning and emotions.
Said Shors: "Sex hormones, like estrogen, have a tremendous effect
on the growth and architecture of the brain."