Scientists have made extraordinary progress in discovering the biological mechanisms underlying human sex, sexual behavior, and gender. In some cases, these discoveries have shaken our understanding of what it means to be a man or a woman, and even whether simple categories like "male" and "female" are scientifically meaningful.

Here are a few of the most significant discoveries made about gender recently. These are described in more detail in my book Sex, Love, and DNA. It’s my hope that with better scientific understanding will come more tolerance of the remarkable human sexual spectrum that we share.

1. Our sex is determined by our body’s chemistry — sometimes, in surprising ways.

The X and Y chromosomes (the “sex chromosomes”) usually determine whether someone is male or female. Women generally have two X chromosomes and men one X and one Y. Yet some people's chromosomes appear to be incompatible with their physical appearance.

It’s possible that a person with certain rare genetic variants in their DNA won’t have the usual biochemical processes that typically lead to male or female development. Consequently, some healthy (though usually infertile) men do not have a Y chromosome, while healthy females can have a Y chromosome.

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These kinds of genetic variations can also lead to individuals developing with some male and some female sexual characteristics. It's been posited that in approximately one out of every 2,000 births, the doctor performing the delivery is unsure of the sex of the newborn. That doesn’t mean anything is wrong with the child. On the contrary, they are simply wonderfully unique, and their individuality should be celebrated.

2. A single gene or protein can change one's sexual orientation.

Laboratory mice can be genetically modified so they’re unable to produce certain enzymes involved in hormone production. These mice are healthy and, to all appearances, normal. However, the females display typical male sexual behavior. They mount other females and prefer to sniff female urine. Physically, they are 100 percent female. Yet disabling a single protein somehow can switch their brain wiring to become “homosexual.”

3. Non-heterosexual behavior is “natural.”

Homosexual animal behavior is not limited to genetically modified laboratory animals. It actually occurs naturally among many animal species — from sheep to birds. Among bonobos, close relatives of chimpanzees, approximately 60 percent of sexual activity occurs between females. Among Laysan albatross, pairs of females may stay together as “socially monogamous” couples for more than 10 years, during which time they return to their shared nests every year to raise their chicks. (They do mate with males to fertilize their eggs, but the male-female pairs don’t stay together long.)

4. Even among people, sexual orientation is at least partly innate.

The strongest evidence comes from comparing identical and nonidentical twins. Such investigations show that, at least among males, homosexuality is much more commonly shared by identical twins than by nonidentical twins. That said, even among identical twins only in 15 percent of the cases were both brothers gay, meaning non-genetic factors clearly play important roles as well.

5. Transsexuality is also influenced by biology.

Transsexuals believe their “true” sex is not that indicated by their sexual anatomy. There is now evidence that a person’s innate biology affects whether or not they feel transsexual. In a few cases, scientists have even found underlying genetic factors. In most cases, though, the biological roots of transsexuality remain unknown.

Nevertheless, studies of transsexual twins show that approximately 33 percent of transsexual identical male twins have a twin who is also transsexual, while only 4.8 percent percent of transsexual nonidentical male twins have a twin who is also transsexual. This would indicate that transsexuality has a strong inherited component.

6. “Normal” doesn’t necessarily mean better.

In science (as well as in the dictionary), the word normal means "typical, usual, or conforming to a standard type." Unfortunately, many people also define normal as "desirable or healthy." In the case of genetics, some atypical DNA variants cause disease. But other rare genetic variants can protect one from heart disease or AIDS, or can lead to unusually strong bones or muscles.

In fact, most rare genetic variations have little or no impact on someone's health. They may, for example, change their hair color or food preferences – or have no observable impact at all. Moreover, scientists have recently discovered that we are all "abnormal" in that each of us has thousands of rare genetic variants.

7. People with atypical sex or gender face challenges primarily from the attitudes of others.

Transgender or homosexual individuals are not "normal," in that they are not typical. Nevertheless — although the biological mechanisms underlying human sexual orientation and gender identity are still poorly understood — biology shows us that they are neither deviant nor in any way unacceptable. The challenges of those with different biology rarely stem from their intrinsic differences. Instead, they stem most often from the hostile attitudes shown to them by people who misunderstand.

Perhaps recognizing that different does not mean "bad, or dangerous," will allow more of us to accept the uniqueness of ourselves and others as a gift, and value one another accordingly.

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