3276. What Genetics Is Teaching Us About Sexuality

By Steven M. Phelps and Robbee Wedow, The New York Times, August 29, 2019

Art: Nicholas Konrad, The New York Times

As researchers in biology and sociology who are also gay men, we have long wondered (and debated) whether sexual orientation has any biological basis. We followed the ascent in the 1990s of the “gay gene” finding — which claimed that male sexual orientation was linked to specific DNA markers — and then watched as that result was called into question. We have wondered whether the two of us, who differ in so many ways, could really trace our common identity to a shared biology.

New data are finally giving us answers.

A study published Thursday in Science looked at the DNA and sexual behavior of nearly 500,000 people. It found that the sex of your sexual partners is, in fact, influenced by your genes. But it also found that it was not possible to predict your sexual behavior from your DNA alone. The study suggested, in other words, that while biology shapes our most intimate selves, it does so in tandem with our personal histories — with the idiosyncratic selves that unfold in a larger cultural and social context.

The researchers, who included one of us (Dr. Wedow), analyzed the genetic markers of people who responded to the question “Have you ever had sex with someone of the same sex?” From these data, the researchers estimated that genetic differences account for roughly one-third of the variation in same-sex behavior. The study also identified several DNA sequence variants associated with having had a same-sex experience.
So, yes, your sex life is influenced by your genes.

This conclusion fits with our personal experiences and intuitions. Sexual desire is typically stable, something we often are aware of from our first longings. Furthermore, one of the several DNA variants identified in Thursday’s study is involved in gonad development, which accords with previous research that links sexual orientation to hormone exposure.

But the study’s findings also complicate the relationship between genetics and sexuality.

For one thing, the results make clear that there is no single biology of sexual behavior. It turns out, for example, that the genes influencing same-sex behavior in females are often different from those that shape behavior in males. It also turns out that the genes associated with having occasional same-sex experiences are unlinked to having exclusively same-sex experiences.

In addition, people who only occasionally have same-sex partners tend to have genetic variants associated with having more sexual partners overall, and with personality traits like “openness to new experience.” In contrast, the study found that exclusively same-sex behavior had little correlation with the biology of personality. For some people, same-sex behavior may be a form of exploration. For many others, it is not.

Researchers since the 1940s have described sexual orientation as a single trait that exists on a scale, ranging from “exclusively heterosexual” to “exclusively homosexual.” But the new study suggests that sexuality is more diverse than that — many different things, rather than one thing in greater or lesser degrees.

Although valuable, the study does have limits. The sample size is enormous by historical standards, but the sample still excludes most minority groups. This not only impairs our ability to make generalizations across people with different ancestries, it also overlooks the cultural variation within and among groups.

Similarly, the study examines only the binary distinction between same-sex and opposite-sex behavior; it does not examine gender (as opposed to biological sex) or the many other varieties of sexual interest. In order to advance science for everyone, we need studies that explore the full diversity of genetics and experience.

It’s also important not to overstate the role of genes. The study found that genetically related people tend to be similar in their behavior, which tells us that sexuality has influences buried somewhere in the DNA. But when the researchers tried to add up the contributions of each DNA variant they examined, they could predict less than 1 percent of the variation among study participants.

So researchers could never predict sexual behavior from DNA alone. In fact, we’ve known this for a while. Even if you have a homosexual identical twin — someone who shares all of your DNA — you are still more likely to be straight than gay.

Many people are wary of genetic research into sexuality because they fear that scientific findings could be used to advance discrimination. They worry that people will attempt to eliminate same-sex behavior using gene-editing technologies like CRISPR or by screening embryos. But the genetic science of sexuality shows us that neither effort would work.

We must also recognize that bigotry needs no data. No facts will sway those who want to police the intimacies of consenting adults. Rather than consign ourselves to ignorance out of fear, we should use these powerful new data ethically and thoughtfully to arrive at a fuller understanding of who we are.
It’s prejudice, not knowledge, that threatens us.

3055. Why Sex Is Not Binary

By Anne Fausto-Sterling, The New York Times, October 25, 2018

No single biological measure unassailably places each and every human into one of two categories — male or female. 

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Two sexes have never been enough to describe human variety. Not in biblical times and not now. Before we knew much about biology, we made social rules to administer sexual diversity. The ancient Jewish rabbinical code known as the Tosefta, for example, sometimes treated people who had male and female parts (such as testes and a vagina) as women — they could not inherit property or serve as priests; at other times, as men — forbidding them from shaving or being secluded with women. More brutally, the Romans, seeing people of mixed sex as a bad omen, might kill a person whose body and mind did not conform to a binary sexual classification.

Today, some governments seem to be following the Roman model, if not killing people who do not fit into one of two sex-labeled bins, then at least trying to deny their existence. This month, Prime Minister Viktor Orbán of Hungary banned university-level gender studies programs, declaring that “people are born either male or female” and that it is unacceptable “to talk about socially constructed genders, rather than biological sexes.” Now the Trump administration’s Department of Health and Human Services wants to follow suit by legally defining sex as “a person’s status as male or female based on immutable biological traits identifiable by or before birth.”

This is wrong in so many ways, morally as well as scientifically. Others will explain the human damage wrought by such a ruling. I will stick to the biological error.

It has long been known that there is no single biological measure that unassailably places each and every human into one of two categories — male or female. In the 1950s the psychologist John Money and his colleagues studied people born with unusual combinations of sex markers (ovaries and a penis, testes and a vagina, two X chromosomes and a scrotum, and more). Thinking about these people, whom today we would call intersex, Dr. Money developed a multilayered model of sexual development.

He started with chromosomal sex, determined at fertilization when an X- or Y-bearing sperm fuses with an X-bearing egg. At least that’s what usually happens. Less commonly, an egg or sperm may lack a sex chromosome or have an extra one. The resultant embryo has an uncommon chromosomal sex — say, XXY, XYY or XO. So even considering only the first layer of sex, there are more than two categories.

And that’s just the first layer. Eight to 12 weeks after conception, an embryo acquires fetal gonadal sex: Embryos with a Y chromosome develop embryonic testes; those with two X’s form embryonic ovaries. This sets the stage for fetal hormonal sex, when the fetal embryonic testes or ovaries make hormones that further push the embryo’s development in either a male or female direction (depending on which hormones appear). Fetal hormonal sex orchestrates internal reproductive sex (formation of the uterus, cervix and fallopian tubes in females or the vas deferens, prostate and epididymis in males). During the fourth month, fetal hormones complete their job by shaping external genital sex — penis and scrotum in males, vagina and clitoris in females.

By birth, then, a baby has five layers of sex. But as with chromosomal sex, each subsequent layer does not always become strictly binary. Furthermore, the layers can conflict with one another, with one being binary and another not: An XX baby can be born with a penis, an XY person may have a vagina, and so on. These kinds of inconsistencies throw a monkey wrench into any plan to assign sex as male or female, categorically and in perpetuity, just by looking at a newborn’s private parts.

Adding to the complexity, the layering does not stop at birth. The adults surrounding the newborn identify sex based on how they perceive genital sex (at birth or from an ultrasound image) and this begins the process of gender socialization. Fetal hormones also affect brain development, producing yet another layer called brain sex. One aspect of brain sex becomes evident at puberty when, usually, certain brain cells stimulate adult male or adult female levels and patterns of hormones that cause adult sexual maturation.

Dr. Money called these layers pubertal hormonal sex and pubertal morphological sex. But these, too, may vary widely beyond a two-category classification. This fact is the source of continuing disputes about how to decide who can legitimately compete in all-female international sports events.

There has been a lot of new scientific research on this topic since the 1950s. But those looking to biology for an easy-to-administer definition of sex and gender can derive little comfort from the most important of these findings. For example, we now know that rather than developing under the direction of a single gene, the fetal embryonic testes or ovaries develop under the direction of opposing gene networks, one of which represses male development while stimulating female differentiation and the other of which does the opposite. What matters, then, is not the presence or absence of a particular gene but the balance of power between gene networks acting together or in a particular sequence. This undermines the possibility of using a simple genetic test to determine “true” sex.

The policy change proposed by the Department of Health and Human Services marches backward in time. It flies in the face of scientific consensus about sex and gender, and it imperils the freedom of people to live their lives in a way that fits their sex and gender as these develop throughout each individual life cycle.

Anne Fausto-Sterling is an emeritus professor of biology and gender studies at Brown University.