Sexual identity may be hard-wired into the brain. Before a developing embryo begins to generate its own hormones, genes are already playing an important role in organizing the brain along gender. Researchers from University of California at Los Angeles studying mice embryos have identified 54 genes in which activity levels vary according to gender.

Eric Vilain, an assistant professor of genetics at UCLA, compared the activity levels of genes in male and female brains in 10-day-old embryonic mice—days before they developed sex organs. He found 18 genes that were more strongly active in male embryonic brains, and 36 that were revved up in female brains.

The finding suggests that genes play an important role in the early development of sexual identity, and probably have much to do with shaping gender in the mammalian brain. "This refutes the notion that hormones are the sole influence in gender identity," says Vilain.

Still, scientists do not know how large a role genetics plays in making the brain male or female. Hormones and environmental factors are certainly a vital part of development.

Vilain plans to investigate how individual genes influence the brain's development.

The findings may also help children born with ambiguous genitalia. Also, it may potentially explain why transgendered people have normal hormonal levels, yet feel they are in the wrong body.

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