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The nuanced science of biological sex

In many societies today, sex has been traditionally defined as immutable binary. In other words, many societies view everyone as belonging to either one of two categories: male or female. People who are male are men or boys. People who are female are women or girls. In such a view, gender is nothing but another word for sex.

While many aspects of this binary are cultural—for example, attitudes about the wearing of skirts or pants differ across cultures and time periods—it is rooted in the belief that biological sex is both an absolute binary and the sole determinant for a person’s gender identity. However, as we learn more about the science of sex determination, we begin to realize that a neat binary is not enough to describe the full picture. The same goes for gender identity, if not more so.

The story of a child named MC illustrates the need for society to adopt a more nuanced view. MC was born with a condition that produces ambiguous genitalia with both ovarian and testicular tissue. When MC was 16 months old and under the care of the state of South Carolina, doctors performed a surgery that effectively assigned MC as female.

However, MC grew up as a boy and has always identified as a boy. Pamela Crawford, MC’s adoptive mother, said MC feels more like a boy and “wants to be a normal boy.” As a result, MC’s adopted parents sued those responsible for performing and signing off on the surgery.

After four years of legal battle, a judge ruled in MC’s favor last year. MC, now 13 years old, will be paid $440,000 in 16 years because of the surgery.

MC’s case, while rare, is not extraordinary. Many people live with conditions that challenge the view that sex is an absolute binary.

The most basic view of sex involves chromosomes. People with XX chromosomes are female. People with XY chromosomes are male.

Surprisingly, the nuances of biological sex begin even at this very basic level.

The X and Y chromosomes contain genes that determine how an embryo develops. One important gene is the SRY gene in the Y chromosome. SRY causes the development of an embryo’s testes, which in turn release hormones called androgens that signal the development of male organs.

This is why some people with XX chromosomes can develop into males if their cells have a fragment of a Y chromosome that contains SRY. Meanwhile, individuals who are XY but whose SRY genes are damaged can develop as females, a condition known as Swyer Syndrome.

People who are XY can develop as females because of a different condition called complete androgen insensitivity (CAIS). People with CAIS have a Y chromosome with a functioning SRY gene. Because of this they have an internal testes that releases androgens. However, because the rest of their cells are not sensitive to androgens, they develop female external genitalia and grow up to be females at puberty.

Such conditions, called differences or disorders of sex development (DSDs), are not as rare as one might think. “Some researchers now say that as many as 1 person in 100 has some form of DSD,” writes Claire Ainsworth for Nature Magazine.

“But beyond this, there could be even more variation,” Ainsworth says. “[Next-generation] DNA sequencing in the past few years have uncovered a wide range of variations in these genes that have mild effects on individuals, rather than causing DSDs.”

“Biologically, it’s a spectrum,” says Eric Vilain, director of the Center for Gender-Based Biology at the University of California, Los Angeles.

The complexity of biological sex does not stop there. “To varying extents, many of us are biological hybrids on a male-female continuum,” the editors of Scientific American write. “Researchers have found XY cells in a 94-year-old woman, and surgeons discovered a womb in a 70-year-old man, a father of four.”

Given that biological sex is itself not an immutable binary, many argue that more so can be said of gender identity.

According to the American Psychological Association (APA), gender identity is “a person’s deeply-felt, inherent sense” of their gender.

While most people are cisgender—they have a gender identity that aligns with their sex at birth—a large number of us are transgender. According to the APA, transgender people have a gender identity that “does not conform to what is typically associated with their sex at birth.”

When the brains of trans persons were scanned by scientists, they found that their brain activities matched their gender rather than their sex assigned at birth. This reveals that gender identity is indeed distinct from biological sex.

As we discover more about sex and gender, there is the hope that we, as a society, become more accepting of the diversity of the human experience.

Topics: Sounds of Science , biological sex
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