Women buy shampoos that promise “silky smoothness.”
Men buy the same “dandruff protection”
shampoos.Â Advertisers have known ““ men and women are
For decades, scientists have attributed these differences to sex
hormones, but a new study by UCLA scientists suggests that gender
identity is hardwired into the brain long before sex hormones are
“Our findings may help answer an important question: Why
do we feel male or female?” said Dr. Eric Vilain, assistant
professor of human genetics, pediatrics and urology at the David
Geffen School of Medicine and principal investigator of the
Working with mice, Vilain and a group of scientists extracted
RNA from the embryonic brains of 10-day-old males and females and
compared levels of activity in different genes.
To their surprise, 54 genes had different levels of
activity.Â About half were more active in females than males, a
fact Vilain said may explain gender-specific behavior
For example, boys play more aggressively, preferring what is
called “rough-and-tumble” activities. Girls, on the
other hand, prefer to have more quiet activities, Vilain said.
Previous studies have shown sex hormones, estrogen and
testosterone, to be the cause of gender identity.
“Hormones organized the brain in a masculine or feminine
fashion,” Vilain said.
However, a growing number of studies have shown hormones could
not fully explain some aspects of sexual identity, he added.
Because sex hormones are produced in 13-day-old embryonic mice,
scientists were able to discount hormones as the cause of varying
levels of genetic activity between the 10-day-old male and female
Scientists genotyped the mice beforehand to determine their
In addition to explaining some behavioral differences, the
varying levels may also explain physiological differences in the
brains of males and females.
Some documented differences include a structure called the
sexually dimorphic nucleus, a cluster of cells found in the
hypothalamus, that is two and a half times bigger in males than
females. In rats, the SDN contributes to mating and territorial
behavior, but its function in humans is unknown.
Another brain structure where size appears to be sex-based is
the corpus callosum, a bundle of nerves that links the right and
left hemispheres of the brain.Â This structure, in addition to
two other linkage structures, is larger and more integrated in
females, possibly explaining the phenomenon known as
Additionally, a female’s brain is much more symmetrical
than a male’s.Â The symmetry may improve communication
between both sides of the brain, leading to better verbal
expressiveness in females, Vilain said.
His team is currently creating genetically modified mice to
discover the specific roles of the genes.
In addition, scientists hope to find if these genes are
responsible for gender identity disorders.
“For instance, you have little boys who do gender
non-conforming activities. They would prefer to hang out with
girls, cross-dress, or put their mother’s shoes on,”
Scientists aren’t sure what is happening in these
instances and in cases of homosexuality and transgender
“Our next step is to look at the genes that we found and
see if there are genetic variations in transgender
individuals,” Vilain said.Â
This research could then possibly help doctors assign a gender
to intersex infants.
However, there is a movement within the intersex community that
condemns surgically modifying a child to fit one gender or
“The intersex movement is saying, “˜Allow us to grow
and discover who and what we are,'” said Ronni Sanlo,
director of the UCLA Lesbian Gay Bisexual Transgender Campus
With further research, Vilain hopes to discover whether
homosexuality and transgender identity are truly choices.
“It’s quite possible that sexual identity and
physical attraction is “˜hardwired’ by the brain,”
Vilain said.Â “If we accept this concept, we must dismiss
the myth that homosexuality is a choice and examine our civil legal
system accordingly,” he added.
The study was published this week in the journal Molecular Brain