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Marc Breedlove vs. transgender people

by Andrea James Reviewed November 12, 2026 9 min read

Marc Breedlove is an American psychologist and anti-transgender activist. Breedlove is closely aligned with anti-trans sexologists active in the late 20th and early 21st centuries. Breedlove was a key promoter of the 2003 anti-trans book The Man Who Would Be Queen by J. Michael Bailey and has published with Bailey. Breedlove and Bailey co-authored a paper that calls trans women who are attracted to men “transgender male androphiles.”

Breedlove frequently publishes on the biology of sexual orientation. Breedlove has researched how steroid hormones shape the developing and adult nervous system, especially the spinal cord and brain. Breedlove is also known for research on prenatal influences on human sexuality.

Background

Stephen Marc Breedlove was born in 1954 in Missouri. After graduating from Springfield Central High School in 1952, Breedlove earned a bachelor’s degree from Yale University in 1976, then attended University of California, Los Angeles, earning a master’s degree, followed by a doctorate in 1982.

Breedlove was a professor of psychology at the notoriously anti-trans psychology department at University of California, Berkeley, from 1982 to ~2002. Breedlove then moved to Michigan State University.

Sexuality research

Breedlove helped identify the spinal nucleus of the bulbocavernosus (SNB), a group of motor neurons in the rat lumbar spinal cord that controls muscles attached to the penis. Breedlove coauthored a 1980 Science paper which found that these neurons accumulate androgens.

In a 1997 Nature paper titled “Sex on the brain,” Breedlove found that sexual experience itself can alter neural structure.

In March 2000, Breedlove and colleagues published an article in Nature titled “Finger-length ratios and sexual orientation.” The study used the ratio of index-finger (2D) to ring-finger (4D) length, which is on average lower in men than in women and is thought to reflect prenatal androgen exposure.

In 2008, Breedlove coauthored a paper on a gastrin-releasing peptide (GRP) system in the lumbar spinal cord that controls reproductive reflexes.

Anti-trans activism

Breedlove is closely involved with a group of sexologists associated with anti-trans journal Archives of Sexual Behavior, which was created in part to study “the prevention of transsexualism.”

In 1995, Dutch researchers published a paper on “a sex difference in the human brain and its relation to transsexuality.” Using postmortem brain tissue, the authors reported that the volume of the central subdivision of the bed nucleus of the stria terminalis (BSTc), a brain area that is essential for sexual behavior, is reportedly larger in men than in women, and a female-sized BSTc was found in “male-to-female transsexuals.” In an introduction to the paper, Breedlove wrote that the observed difference might reflect hormone treatment rather than something present before transition, concluding the study would be “far from the final word on the subject.”

Breedlove advocates for the “gametic definition” of sex. In a 2025 commentary, Breedlove wrote:

“[E]volution has imposed a strict dichotomy on animal gametes and we find this distinction inviolate. However, after that, characteristics associated with these two kinds of gametes often present as a spectrum. [… We are left] with a perpetually unresolvable debate about how to classify a person’s sex or if we should even try.”

[…] On a population level, for all sexually reproducing animals on this planet, there are two sexes as determined by size of the gametes they produce. When one digs further than that, there are finer gradations and differences both between species and within them, but broadly, we find it most useful to divide the sexes by gametes. Importantly, the characteristics associated with one gamete or the other at the population level often present as a continuum and there have always been some individuals bearing both female- and male-typical characteristics. Even so, we see no utility in trying to devise complex guidelines to split sexes into more than two categories, as it could be done, in theory, ad infinitum.

Breedlove believes the distinction between sex and gender has value, and that gender (but not sex) is socially constructed:

“For gender, the case is quite different. The nature of human social behavior means that the (assumed) presence of one gamete or the other brings along additional expectations of comportment, appearance, and attitude that extend beyond the mating and courtship behaviors seen in other animals. The social categories associated with gametes have material implications for the realities of both individuals and societies at large. Gender as a term captures the social distinction that explains how female and male translate to women/girls and men/boys and the expectations for how one ought to behave based on placement in those categories. Gender as a social rather than biological distinction, separate but normatively associated with sex, allows us to explain why these expectations vary so greatly across cultures and can change across time within a given culture, even though the basic biology of sexual reproduction was fixed long before our species arose. It also explains why some cultures, but not others, recognize more than two genders.

[… W]hat compelling interest would an egalitarian government or society have to inquire about any given citizen’s sex, much less their gender? Medical professionals may need to know an individual’s sex to provide optimal care, but what difference would (or should) it make for driving a car, paying taxes, obeying laws, or enjoying the rights of citizenship? What difference would (or should) it make to the kinds of jobs, education, and resources people have access to?

The Sex Files (2000)

Breedlove was featured on an episode of The Sex Files titled “Homosexuality” with Bailey and the usual suspects:

The Sex Files
HOMOSEXUALITY
IN THIS EPISODE

Why are some people gay? That’s the $64,000 question – at least in the scientific community. Is it something genetically predetermined? Or does environment have an impact on whether an individual turns out to be gay or lesbian? These questions are beginning to be probed in ways that might finally be leading to an answer, and the Sex Files has interviewed the foremost authorities on the topic to uncover some of those scientific clues:

  • Dr. Devendra Singh, University of Texas psychologist specializing in the evolutionary significance of human physical attractiveness
  • Dr. Ken Zucker, head of the Child and Adolescent Gender Identity Clinic at the University of Toronto’s Clarke Institute of Psychiatry
  • Dr. Ray Blanchard, head of the Clinical Sexology program at the University of Toronto’s Clarke Institute of Psychiatry
  • Dr. Michael Bailey, professor of psychology at Northwestern University in Illinois and specialist in the genetics and environment of sexual orientation
  • Dr. Marc Breedlove, professor* specialising in the sexual differentiation of the brain.

* The original episode guide described Breedlove as a “professor of psychology at UCLA.” Dr. Breedlove stated in a 2008 email to this site “I am not, and have never been, a professor of psychology or of anything else at UCLA.” Breedlove earned a doctorate at UCLA but taught at UC Berkeley before taking an appointment at Michigan State.

The Man Who Would Be Queen (2003)

Breedlove was one of the first to write an Amazon shill review for the anti-trans book The Man Who Would Be Queen by J. Michael Bailey. On April 30, 2003, Breedlove wrote:

What’s the fuss about? Read the book, think for yourself

Why this vehement response to this terrific book? Because Bailey describes male-to-female transsexuals who report an experience that is quite different from the familiar “a woman trapped in a man’s body”. Bailey never casts doubt that there are such people, in fact he interviews and describes several. But he finds that not all M2F transsexuals fit that mold. So the fuss you’re reading in these reviews are from M2F transsexuals who refuse to acknowledge that other M2F transsexuals might have a different experience than their own. There’s no reason to think the women Bailey interviewed would have been lying to him, and why isn’t their experience as valid as yours, mine or that of other transsexuals?

So get past all the landmines the critics are trying to use to deflect you from reading a thought-provoking, honest and entirely sympathetic view of the fascinating phenomenon of transsexuality.

By the way, it’s a great read, not at all stodgy. I promise you the pages will fly by.

60 Minutes segment (2006)

In 2006, Lesley Stahl presented a widely criticized segment on sexual orientation for 60 Minutes. Participants included J. Michael Bailey, Marc Breedlove, and Gerulf Rieger.

Whom You Love (2014)

In 2012 Breedlove launched a failed crowdfunding campaign for a film called Whom You Love: the biology of sexual orientation. The project was then relaunched and reached half its original funding goal. In 2014, Breedlove released a series of YouTube videos on a channel with that name, featuring many key anti-trans activists in academia.

References

Shorman, Jonathan (June 22, 2013). Neuroscientist and Springfield native shares view on sexual orientation. Springfield News-Leader http://www.news-leader.com/article/20130622/NEWS01/306220007/Springfield-born-neuroscientist-shares-view-on-sexual-orientation [archive]

Staff report (December 5, 2012). Whom You Love: ‘We’re angels made out of dirt.’ Lansing City Pulse https://lansingcitypulse.com/article-8189-Whom-You-Love.html [archive]

Schorn, Daniel (March 9, 2006). The Science Of Sexual Orientation. CBS News / 60 Minutes https://www.cbsnews.com/news/the-science-of-sexual-orientation/

Hegarty, P. (2003). Pointing to a crisis? What finger-length ratios tell us about the construction of sexuality. Radical Statistics, 83. https://www.radstats.org.uk/no083/Hegarty83.pdf

Beneke, Timothy (September 22, 2000). Sex on the Brain: A Conversation with Biological Psychologist Marc Breedlove.

McBroom, Patricia (June 21, 1999). UC Berkeley researchers find first evidence in mammals that sex hormones can change the size of a brain structure. https://newsarchive.berkeley.edu/news/media/releases/99legacy/6-21-1999.html

Suplee, Curt (November 1, 1995). Possible transsexual brain trait found. Washington Post https://www.washingtonpost.com/archive/politics/1995/11/02/possible-transsexual-brain-trait-found/0c9cf0e8-2182-4f68-8cce-2367ec7c7ca9/

Anti-trans coverage

Press release (April 25, 2016). Sexual Orientation, Controversy, and Science. Association for Psychological Science https://www.psychologicalscience.org/publications/sexual_orientation.html

Selected writing by Breedlove

Trevino-Fica, L., & Breedlove, S. M. (2025). Lumping Sexes and Splitting Genders. Archives of Sexual Behavior, 54(10), 3917–3920. https://doi.org/10.1007/s10508-025-03338-5

Breedlove, S. M. (2017). Prenatal Influences on Human Sexual Orientation: Expectations versus Data. Archives of Sexual Behavior, 46(6), 1583–1592. https://doi.org/10.1007/s10508-016-0904-2

Bailey, J. M., Vasey, P. L., Diamond, L. M., Breedlove, S. M., Vilain, E., & Epprecht, M. (2016). Sexual Orientation, Controversy, and Science. Psychological Science in the Public Interest, 17(2), 45–101. https://doi.org/10.1177/1529100616637616

Sakamoto, H., Matsuda, K.-I., Zuloaga, D. G., Hongu, H., Wada, E., Wada, K., Jordan, C. L., Breedlove, S. M., & Kawata, M. (2008). Sexually dimorphic gastrin releasing peptide system in the spinal cord controls male reproductive functions. Nature Neuroscience, 11(6), 634–636. https://doi.org/10.1038/nn.2126

Williams, T. J., Pepitone, M. E., Christensen, S. E., Cooke, B. M., Huberman, A. D., Breedlove, N. J., Breedlove, T. J., Jordan, C. L., & Breedlove, S. M. (2000). Finger-length ratios and sexual orientation. Nature, 404(6777), 455–456. https://doi.org/10.1038/35006555

Breedlove, S. M. (1997). Sex on the brain. Nature 389, 801 (1997). https://doi.org/10.1038/39764

Zhou, J.-N., Hofman, M. A., Gooren, L. J. G., & Swaab, D. F. (1995). A sex difference in the human brain and its relation to transsexuality. Nature, 378(6552), 68–70. https://doi.org/10.1038/378068a0

Breedlove, S. M. (1994). Sexual Differentiation of the Human Nervous System. Annual Review of Psychology, 45(1), 389–418. https://doi.org/10.1146/annurev.ps.45.020194.002133

Breedlove, S. M., & Arnold, A. P. (1980). Hormone accumulation in a sexually dimorphic motor nucleus of the rat spinal cord. Science, 210(4469), 564–566. https://doi.org/10.1126/science.7423210

Resources

Wikipedia (en.wikipedia.org)

Marc Breedlove (marcbreedlove.com)

Whom You Love (whomyoulove.com) [archive]

Michigan State University (msu.edu)

  • Stephen Marc Breedlove
  • msu.edu/honoredfaculty/directory/breedlove-marc-stephen.html
  • Breedlove’s page
  • msu.edu/~breedsm/mb.htm [archive]
  • CV
  • https://neuroscience.natsci.msu.edu/people/faculty/breedlove_cv.pdf [not archived]

YouTube (youtube.com)

Facebook (facebook.com)

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