Genes Organize Male, Female Brain Differently
Dwayne Hunter
Betterhumans Staff
Monday, October 20, 2003, 4:59:41 PM CT

More than 50 genes have been linked to the organization of female and male
brains, refuting 30 years of scientific theory that sexuality is entirely
determined by hormones, offering a genetic basis for gender assignment of
babies born with ambiguous sexual organs and undermining the notion that
homosexuality and gender dysphoria are matters of choice.

Since the 1970s, scientists have believed that the hormones estrogen and
testosterone were wholly responsible for sexually organizing the brain-a
fetal brain under the influence of more testosterone became male and under
the influence of more estrogen became female.

But a study conducted at the David Geffen School of Medicine and the Mattel
Children's Hospital, part of the University of California Los Angeles,
indicates that hormones cannot explain everything about the sexual
differences between male and female brains.

More than 50 points of difference

Eric Vilain, assistant professor of human genetics and urology at David
Geffen, says that sexual identity is rooted in every person's biology before
birth and springs from a variation in the genome.

Using two genetic testing methods that compared genes in male and female
brains in embryonic mice-long before the animals developed sex organs-Vilain
and colleagues explored whether genetic influences could explain brain

The researchers found 54 genes in different amounts in male and female mouse
brains prior to hormonal influences. Eighteen of the genes were produced at
higher levels in the male brains and 36 were produced at higher levels in
the female brains.

"We didn't expect to find genetic differences between the sexes' brains,"
says Vilain. "But we discovered that the male and female brains differed in
many measurable ways, including anatomy and function."

Combined influence

In one example, the team found that two hemispheres of the brain appeared
more symmetrical in females than in males. Villain believes the symmetry may
improve communication between both sides of the brain-an anatomical
difference that may explain why women can sometimes articulate their
feelings more easily than men.

The research implies that genes account for some of the differences between
male and female brains, and that one's genes, hormones and environment exert
a combined influence on sexual brain development.

"From previous studies, we know that transgender persons possess normal
hormonal levels," says villain. "Their gender identity likely will be
explained by some of the genes we discovered."

Villain expects the discovery to lend credence to the idea that gender
dysphoria-feeling that you have been born into the body of the wrong sex-has
biological origins.

Gender assignment and homosexuality

The findings may also ease the plight of parents with children who have
ambiguous genitalia and help physicians assign gender to the children with
greater accuracy.

Mild cases of malformed genitalia occur in one percent of all births and
severe cases occur in about one in every 3,000 births.

"If physicians could predict the gender of newborns with ambiguous genitalia
at birth, we would make less mistakes in gender assignment," said Vilain.

Vilain also proposes that the findings help explain the origin of

"It's quite possible that sexual identity and physical attraction is
'hardwired' by the brain," he says. "If we accept this concept, we must
dismiss the myth that homosexuality is a 'choice' and examine our civil
legal system accordingly."

The scientists will pursue further studies to distinguish specific roles in
the brain's sexual maturation for each of the 54 different genes they

The identification of the 54 genes is reported in the journal Molecular
Brain Research