Sex Is a Spectrum, and Science Keeps Saying So
Intersex is common in birds, according to a new study.
Updated August 20, 2025 | Reviewed by Lybi Ma
Science has a way of nudging us out of our certainties. What once seemed fixed can, with the next experiment or field study, turn out to be more complicated. Few areas have shifted more dramatically than our understanding of sex in recent decades. The latest example comes not from humans but from birds.
In a new study in Biology Letters , scientists examined nearly 500 wild birds from five Australian species—kookaburras, magpies, crested pigeons, rainbow lorikeets, and scaly-breasted lorikeets. They compared each bird's genetic sex, determined by DNA markers, with its reproductive anatomy. The surprise was hard to miss; about 3 to 6 percent showed a mismatch. Genetically female birds (ZW) sometimes had male gonads, and in one extraordinary case, a genetically male kookaburra (ZZ) had a stretched oviduct and appeared to have laid an egg ( Hall and colleagues, 2025 ). For birds—where sex chromosomes are the reverse of mammals, ZZ for males and ZW for females—it was a striking discovery.
This study shows that what scientists call the "three G's"— genes , gonads, and genitals—don't always match. When they don't, individuals fall into what scientists broadly call intersex. Humans are no exception, as the Princeton anthropologist Agustín Fuentes emphasizes in his recent book Sex Is a Spectrum . (See my review of the book here .)
Intersex conditions are already well documented in mammals, especially in livestock and laboratory animals. Cattle, horses, and mice, for example, sometimes produce individuals whose chromosomes and gonads don't align. On the molecular level, one well-known cause is when the Sry gene, which normally triggers testis development on the Y chromosome, ends up on an X chromosome . That can produce an XX male. The reverse can also happen: XY individuals lacking a functional Sry gene, or with downstream mutations, may develop as females. Many other genetic and developmental routes can lead to bodies that don't fit the binary ( Prum, 2023 ; Reyes and colleagues, 2023 ).
Among humans, the most common condition that brings people into the intersex spectrum is late-onset congenital adrenal hyperplasia (CAH). It is usually caused by a partial deficiency of the enzyme 21-hydroxylase. Depending on how strictly it is defined, late-onset CAH may affect up to 1.5 percent of the population ( Fausto-Sterling, 2020 ), or 4.2 percent among women based on the symptoms of androgen excess ( Carmina and colleagues, 2017 ).
Unlike the classic, life-threatening form of the condition, this variant usually doesn't disturb salt balance, but it does cause the adrenal glands to produce more androgens than usual. In women, this can mean irregular periods, infertility , acne, or male-pattern hair growth. In men, the signs are subtler and sometimes only show up during fertility testing. In children, the condition may appear as early puberty , rapid growth, and ultimately shorter adult height.
Taken together, the numbers are striking. A conservative estimate suggests that about 1 percent of all humans have some form of intersex variation. That is roughly 80 million people worldwide—about the population of Germany or twice the population of California. So, far from rare, intersex conditions appear across species and in many contexts, from the Australian bush to human clinics.
What today's science shows is that sex is not a simple binary but a complex interplay among chromosomes, gonads, hormones , and anatomy. The categories of male and female capture much of biological reality, but not all of it. Every discovery—whether a kookaburra that lays an egg against its genes or a person whose chromosomes and body diverge—reinforces this point. Sex is not a switch. It is a spectrum, and as science advances, that spectrum, together with the reasons behind it, only becomes clearer.
Carmina, E., Dewailly, D., Escobar-Morreale, H. F., Kelestimur, F., Moran, C., Oberfield, S., Witchel, S. F., & Azziz, R. (2017). Non-classic congenital adrenal hyperplasia due to 21-hydroxylase deficiency revisited: An update with a special focus on adolescent and adult women. Human Reproduction Update, 23 (5), 580–599.
Fausto-Sterling, A. (2020). Sexing the body: Gender politics and the construction of sexuality . Basic Books.
Hall, C. A., Conroy, G., Jelocnik, M., Kasimov, V., Gillet, A., Portas, T., ... & Potvin, D. A. (2025). Prevalence and implications of sex reversal in free-living birds. Biology Letters , 21 (8), 20250182.
Prum, R. O. (2023). Performance all the way down: Genes, development, and sexual difference . University of Chicago Press.
Reyes, A. P., León, N. Y., Frost, E. R., & Harley, V. R. (2023). Genetic control of typical and atypical sex development. Nature Reviews Urology, 20 (7), 434–451.
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Lixing Sun, Ph.D. , is a distinguished research professor in behavior and evolution at Central Washington University.
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