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Biology of Sexual Identity and Orientation – Part 2 December 22, 2009

Posted by Geekgirl in Uncategorized.

Summarized from Sexual Hormones and the Brain: An Essential Alliance for Sexual Identity and Sexual Orientation

For many individuals, their anatomy, gender identity and sexual orientation fit the mold of heterosexual and cisgender. What does this mean? If a person has male genitals, they usually identify as male and like girls. For a long time, we have considered this normal and have used negative, hurtful words to describe humans who are gay, lesbian, bisexual or transgender. We’ve seen it as an illness, a choice, a sin, or just plain freaks.

Most of us don’t realize that our sense of sexuality extends beyond our genitals. Our brain has a gender identity that tells us if we are male or female. Or maybe in between. The same for our sexual attraction. Some people are attracted to members of the opposite sex, the same sex or both.

So how do we know that gender identity exists inside the brain? Let’s tackle this section of the paper  first.

Before this paper gets into the science, it shares a sad story to help educate us. The story of John – Joan — John (the case of David Reimer). As an 8 month old, David underwent surgery and the surgery went wrong. He lost his penis. The doctors made the decision to remove his testicles and treat him with hormones to turn him into a girl. It was believed at the time that a child becomes male or female solely by how they are raised. (This was an idea that feminists really liked, remember?) His parents raised him as a girl. Well, this story has a sad ending. In adulthood, he changed back to male, married and adopted children. The trauma caused him to commit suicide in 2004. Even with exposure to hormones, his brain had been programmed during development to be a boy.

A little vocabulary first. When a male differs from a female, that is called sexual dimorphism. Morphism comes from the word morphology, which is similar to anatomy. Appearance or shape. Sexual dimorphism just means there are two different appearances and they are related to gender. Our anatomy is sexually dimorphic – yet some individuals are born with ambiguous genitalia. Notice the photo that shows the range from a “normal” boy to a “normal” girl. I put normal in quotes because it’s a very loaded word. All of these are biologically normal. It is just that the genitals that are more ambiguous occur less frequently.

Our brains are also different depending upon our sex. So how do these differences come about?

During fetal development, testicles and ovaries develop in the sixth week of pregnancy. This development starts with the SRY gene on the Y chromosome. The production of testosterone is essential for the development of male genitals. Without testosterone, the fetus develops as a female.

That is when our anatomy develops. But what about our sexual identity? It turns out there is another stage. Sexual differentiation in the brain starts between 12 and 18 weeks of pregnancy, when testosterone levels surge in boys, then again in weeks 34 through 41 of pregnancy, when testosterone levels of boys are ten times higher than girls.

Sexual differential of the brain requires more than hormones. At least 50 genes are part of the development of differences in the brains of male and female mice.

In humans, sexual identity and orientation involves a direct effect of testosterone on the developing brain. For example, a condition called ‘Complete Androgen Insensitivity Syndrome’ or CAIS is caused by mutations in the gene for the androgen receptor. Testosterone is a type of androgen. An androgen receptor is the molecule on the cell’s surface that allows the testosterone into the cell. If that receptor doesn’t function properly, the “door” is locked and testosterone cannot enter the cell. This is why some individuals who are born XY develop as girls, both their genitals and their brains. In other words, they look anatomically like girls and they feel heterosexual. But individuals who have Partial Androgen Insensitivity can often feel like they are male even if they are anatomically female.

Another condition occurs when a male fetus has a deficiency in certain enzymes, either 5alpha reductase-2 or 17beta-hydroxysteroid dehydrogenase-3. These enzyme deficiencies prevent testosterone from being converted to dihydrotestosterone, a necessary step. Then, a girl with a large clitoris is born. These children are usually raised as girls. However, when these children hit puberty, testosterine production goes up and the child begins to masculinize. Approximately 60% will anatomically change into boys and identify as heterosexual.

Some boys are born with a partly or wholly absent penis and are usually changed into girls immediately after birth. A survey showed that in adulthood, only 47% continued to live as girls.

Coming up in part 3 – Transsexuality and the Brain – What can we see in the brain of transsexuals?



1. Tweets that mention Biology of Sexual Identity and Orientation – Part 2 « LGBT Latest Science -- Topsy.com - December 24, 2009

[…] This post was mentioned on Twitter by sexgenderbody, Trans-Ponder. Trans-Ponder said: Biology of Sexual Identity and Orientation – Part 2 http://tinyurl.com/yc3u9lu – TJ […]

2. shashank - January 31, 2010

Here is a link to more information about the genetics of 17-Beta Hydroxysteroid Dehydrogenase 3 Deficiency that was prepared by our genetic counselor and which has links to some useful resource for those dealing with this condition: http://www.accessdna.com/condition/17-Beta_Hydroxysteroid_Dehydrogenase_3_Deficiency/702. There is also a number listed for anyone who wants to speak to a genetic counselor by phone. I hope it helps. Thanks, AccessDNA

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