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Women buy shampoos that promise “silky smoothness.” Men buy the same “dandruff protection” shampoos. Advertisers have known – men and women are different.

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 even developed.

“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 study. 

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. About half were more active in females than males, a fact Vilain said may explain gender-specific behavior differences.

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 mice.

Scientists genotyped the mice beforehand to determine their sexes.

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. This structure, in addition to two other linkage structures, is larger and more integrated in females, possibly explaining the phenomenon known as “women’s intuition.”

Additionally, a female’s brain is much more symmetrical than a male’s. 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,” Vilain said. 

Scientists aren’t sure what is happening in these instances and in cases of homosexuality and transgender sexuality.

“Our next step is to look at the genes that we found and see if there are genetic variations in transgender individuals,” Vilain said. 

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 another.

“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 Resource Center.

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. “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 Research.