(TERF stands for “trans-exclusionary radical feminist.” The term can be useful for making a distinction with radical feminists who do not share the same position, but those at whom it is directed consider it a slur.)

Abusive posts proliferated on Twitter and, especially, Tumblr. One read, “/kill/terfs 2K14.” Another suggested, “how about ‘slowly and horrendously murder terfs in saw-like torture machines and contraptions’ 2K14.” A young blogger holding a knife posted a selfie with the caption “Fetch me a terf.” Such threats have become so common that radical-feminist Web sites have taken to cataloguing them. “It’s aggrieved entitlement,” Lierre Keith told me. “They are so angry that we will not see them as women.” Keith is a writer and an activist who runs a small permaculture farm in Northern California. She is forty-nine, with cropped pewter hair and a uniform of black T-shirts and jeans. Three years ago, she co-founded the ecofeminist group Deep Green Resistance, which has some two hundred members and links the oppression of women to the pillaging of the planet.

D.G.R. is defiantly militant, refusing to condemn the use of violence in the service of goals it considers just. In radical circles, though, what makes the group truly controversial is its stance on gender. As members see it, a person born with male privilege can no more shed it through surgery than a white person can claim an African-American identity simply by darkening his or her skin. Before D.G.R. held its first conference, in 2011, in Wisconsin, the group informed a person in the process of a male-to-female transition that she couldn’t stay in the women’s quarters. “We said, That’s fine if you want to come, but, no, you’re not going to have access to the women’s sleeping spaces and the women’s bathrooms,” Keith told me.

Last February, Keith was to be a keynote speaker at the Public Interest Environmental Law Conference, at the University of Oregon, in Eugene, but the student government voted to condemn her, and more than a thousand people signed a petition demanding that the address be cancelled. Amid threats of violence, six policemen escorted Keith to the lectern, though, in the end, the protest proved peaceful: some audience members walked out and held a rally, leaving her to speak to a half-empty room.

Keith had an easier time at Radfems Respond, where she spoke on the differences between radicalism and liberalism. Two gender-bending punk kids who looked as if they might be there to protest left during the long opening session, on prostitution. A men’s-rights activist showed up—he later posted mocking clips from a video that he had secretly made—but said nothing during the sessions. Several trans women arrived and sat at the back, but, in fact, they were there to express solidarity, having decided that the attacks on radical feminists were both out of control and misguided. One of them, a thin, forty-year-old blonde from the Bay Area, who blogs under the name Snowflake Especial, noted that all the violence against trans women that she’s aware of was committed by men. “Why aren’t we dealing with them?” she asked.

Despite that surprising show of support, most of the speakers felt embattled. Heath Atom Russell gave the closing talk. A stocky woman, with curly turquoise hair and a bluish stubble shadow on her cheeks, she wore a T-shirt that read “I Survived Testosterone Poisoning.” At twenty-five, she is a “detransitioner,” a person who once identified as transgender but no longer does. (Expert estimates of the number of transitioners who abandon their new gender range from fewer than one per cent to as many as five per cent.)

Russell, a lesbian who grew up in a conservative Baptist family in Southern California, began transitioning to male as a student at Humboldt State University, and was embraced by gender-rights groups on campus. She started taking hormones and changed her name. Then, in her senior year, she discovered “Unpacking Queer Politics” (2003), by Sheila Jeffreys, which critiques female-to-male transsexualism as capitulation to misogyny.

At first, the book infuriated Russell, but she couldn’t let go of the questions that it raised about her own identity. She had been having heart palpitations, which made her uneasy about the hormones she was taking. Nor did she ever fully believe herself to be male. At one point during her transition, she hooked up with a middle-aged trans woman. Russell knew that she was supposed to think of herself as a man with a woman, but, she said, “It didn’t feel right, and I was scared.” Eventually, she proclaimed herself a woman again, and a radical feminist, though it meant being ostracized by many of her friends. She is now engaged to a woman; someone keyed the word “dyke” on her fiancée’s car.

Russell appears in Sheila Jeffreys’s new book, “Gender Hurts: A Feminist Analysis of the Politics of Transgenderism.” Jeffreys, who is sixty-six, has short silver hair and a weathered face. She has taught at the University of Melbourne for twenty-three years, but she grew up in London, and has been described as the Andrea Dworkin of the U.K. She has written nine previous books, all of which focus on the sexual subjugation of women, whether through rape, incest, pornography, prostitution, or Western beauty norms. Like Dworkin, she is viewed as a heroine by a cadre of like-minded admirers and as a zealot by others. In 2005, in an admiring feature in the Guardian, Julie Bindel wrote, “Jeffreys sees sexuality as the basis of the oppression of women by men, in much the same way as Marx saw capitalism as the scourge of the working class. This unwavering belief has made her many enemies. Postmodern theorist Judith Halberstam once said, ‘If Sheila Jeffreys did not exist, Camille Paglia would have had to invent her.’ ”

In eight brisk chapters (half of them written with Jeffreys’s former Ph.D. student Lorene Gottschalk), “Gender Hurts” offers Jeffreys’s first full-length treatment of transgenderism. Ordinarily, Jeffreys told me, she would launch the publication of a new book with an event at the university, but this time campus security warned against it. She has also taken her name off her office door. She gave a talk in London this month, but it was invitation-only.

In the book, Jeffreys calls detransitioners like Russell “survivors,” and cites them as evidence that transgenderism isn’t immutable and thus doesn’t warrant radical medical intervention. (She considers gender-reassignment surgery a form of mutilation.) “The phenomenon of regret undermines the idea that there exists a particular kind of person who is genuinely and essentially transgender and can be identified accurately by psychiatrists,” she writes. “It is radically destabilising to the transgender project.” She cites as further evidence the case of Bradley Cooper, who, in 2011, at the age of seventeen, became Britain’s youngest gender-reassignment patient, then publicly regretted his transition the next year and returned to living as a boy. Jeffreys is especially alarmed by doctors in Europe, Australia, and the United States who treat transgender children with puberty-delaying drugs, which prevent them from developing unwanted secondary sex characteristics and can result in sterilization.

Throughout the book, Jeffreys insists on using male pronouns to refer to trans women and female ones to refer to trans men. “Use by men of feminine pronouns conceals the masculine privilege bestowed upon them by virtue of having been placed in and brought up in the male sex caste,” she writes. To her critics, the book becomes particularly hateful when she tries to account for the reality of trans people. Explaining female-to-male transition is fairly easy for her (and for other radical feminists): women seek to become men in order to raise their status in a sexist system. Heath Atom Russell, for example, is quoted as attributing her former desire to become a man to the absence of a “proud woman loving culture.”