Human biological sex is often thought of in binary terms male or female but the science tells a more complex story. A wide range of biological, genetic and developmental factors shape what we understand as sex. For millions of people, these factors don’t align with traditional male or female classifications. This complexity is not just theoretical it has real-life implications for medicine, law, identity documents and social life.
Table of Contents:
- How Sam Sharpe and Anna Biason-Lauber describe sex development in humans
- Chromosomal patterns beyond typical XX and XY
- Intersex traits and biological variation
- The role of hormones and puberty in shaping sex traits
- Biological sex as a continuum, not a binary
How Sam Sharpe and Anna Biason-Lauber describe sex development in humans
Sexual development in humans begins in the womb, starting several weeks after conception. During this early stage, reproductive tissues form without distinct male or female characteristics. Around the sixth week of gestation, precursor cells appear, which will later differentiate into ovaries or testes. However, at this stage, these cells are still indistinguishable.
Around the eighth week, the presence or absence of certain hormones begins to guide this differentiation. In individuals with testes, cells begin producing testosterone, initiating the development of male reproductive structures. However, sperm production does not occur until puberty, when testosterone levels rise again. In contrast, ovaries do not produce sex hormones during fetal development. Female reproductive structures such as the uterus and vagina form without hormonal input. The eggs present in ovaries remain dormant until puberty.
This gradual and varied process means that sexual characteristics evolve over time, not instantly at birth. For example, hormone levels and their effects vary greatly between individuals, even within the same sex.
Chromosomal patterns beyond typical XX and XY
Sex chromosomes, often considered the definitive markers of male or female identity, are more varied than typically assumed. While most females have two X chromosomes and most males have one X and one Y, numerous variations challenge this norm.
Turner syndrome occurs in individuals with a single X chromosome. These individuals are often considered female, have a uterus, but typically lack functional ovaries. This affects 1 in every 2,000 to 2,500 female births.
Klinefelter syndrome affects about 1 in 650 male births. These individuals have at least two X chromosomes and one Y chromosome. They have testes and penises but do not produce sperm.
Other chromosomal patterns involve the presence of an SRY gene (which influences male development) on an X chromosome. This can result in people with two X chromosomes developing male characteristics, despite lacking a complete Y chromosome.
These examples demonstrate that sex cannot be defined solely by chromosome pairs. Variations in key genes or gene activity can override traditional XX/XY definitions.
Intersex traits and biological variation
Intersex individuals are born with physical traits that do not fit standard definitions of male or female. These can involve:
- Mixed reproductive organs
- Hormonal differences
- Atypical chromosomal combinations
- External genitalia that don’t match common male or female norms
According to InterAct, approximately 1.7% of the population is intersex a frequency comparable to red hair. Some people may be unaware they are intersex until medical testing in adulthood. Others discover it during childhood due to ambiguous genitalia or hormone-related conditions.
In many countries, intersex infants are subject to surgeries intended to align their bodies with one sex, often decided by the parents. These procedures can involve modifying external genitalia or removing internal reproductive tissue. Additionally, intersex individuals may require lifelong hormone therapy to maintain physical health. These hormones influence bone strength, skin condition and puberty progression.
The role of hormones and puberty in shaping sex traits
While sex development begins in utero, major traits associated with male or female identities only fully emerge during puberty. Hormones such as estrogen and testosterone increase during this period and trigger a wide array of changes. These include growth spurts, secondary sexual characteristics, and fertility.
However, the body's ability to respond to these hormones varies. Some people have gene variants that limit their sensitivity to testosterone. In such cases, individuals with a Y chromosome and testes may develop female-typical features. These cases further highlight the variability of sex traits.
Puberty is also the time when dormant reproductive cells eggs and sperm begin to mature. But not everyone produces these cells. Variants in numerous genes can prevent gamete development. This includes people with typical male or female bodies who are nonetheless infertile due to genetic factors.
Biological sex as a continuum, not a binary
From genetics to hormone sensitivity, sex in humans exists on a spectrum. Binary labels such as "male" and "female" do not capture the full complexity of biological variation. Many people do not fit neatly into these categories, and attempts to impose rigid definitions often result in confusion or exclusion.
Daily life is shaped by sex markers from restrooms to passports to sports. Yet most people never undergo testing for chromosomes or gamete production, which undercuts definitions based on these factors. Instead, sex is typically assigned at birth based on external appearance, which does not always reflect internal biology.
Science increasingly recognizes that no single trait whether it be chromosomes, gametes or hormones can reliably determine sex across all individuals. With an estimated 1 in 60 people falling outside traditional definitions in one way or another, understanding sex as a diverse and multifaceted trait is essential for accurate science and inclusive policy.
Source: Science News Explores