In detailing why she's "worried about the new trans activism," Rowling discussed her personal history as a domestic abuse and sexual assault survivor.
"I'm mentioning these things now not in an attempt to garner sympathy," she wrote, "but out of solidarity with the huge numbers of women who have histories like mine, who've been slurred as bigots for having concerns around single-sex spaces."
Rowling addressed the changing process by which a man can transition into legally being a women as part of her concerns.
"A man who intends to have no surgery and take no hormones may now secure himself a Gender Recognition Certificate and be a woman in the sight of the law," she wrote.
Rowling also discussed a draft bill presented to the Scottish government designed to make it easier for trans people to obtain a gender recognition certificate.
While wanting "trans women to be safe," Rowling wrote that, "at the same time, I do not want to make natal girls and women less safe. When you throw open the doors of bathrooms and changing rooms to any man who believes or feels he's a woman – and, as I've said, gender confirmation certificates may now be granted without any need for surgery or hormones – then you open the door to any and all men who wish to come inside. That is the simple truth."
She wrote that she spent much of Saturday "in a very dark place inside my head, as memories of a serious sexual assault I suffered in my twenties recurred on a loop. That assault happened at a time and in a space where I was vulnerable and a man capitalised on an opportunity. I couldn't shut out those memories and I was finding it hard to contain my anger and disappointment about the way I believe my government is playing fast and loose with womens and girls' safety."
Rowling, an ex-teacher, said her interest in education, safeguarding and the huge increase in the number of girls transitioning -- citing a 4400% increase in the UK -- are also part of her reasons for speaking out.
She also detailed how she felt "mentally sexless" in her own youth and that she has "wondered whether, if I'd been born 30 years later, I too might have tried to transition. The allure of escaping womanhood would have been huge. I struggled with severe OCD as a teenager. If I'd found community and sympathy online that I couldn't find in my immediate environment, I believe I could have been persuaded to turn myself into the son my father had openly said he'd have preferred."
In response, many trans-activists have further criticised Rowling for using her platform to make the lives of trans men and women more difficult.
"JK Rowling's comments not only undermine the core values of the Harry Potter series -- they are flat-out dangerous," wrote Elliott Kozuch, from the Human Rights Campaign, America's largest LGBTQ civil rights organization. "In 2019, at least 26 transgender and gender non-conforming people were killed in the U.S. The majority of these deaths were Black transgender women."
Others were upset by her citation of a controversial 2018 study by Lisa Littman, a study many researchers and trans-activists have been heavily critical of.
Andrew James Carter, the CEO of Podium -- a social network committed to stamping out misinformation -- put together a thread that attempts to add context to Rowling's claims, which Carter calls "half-truths and transphobic dogwhistles".
"Abuse is deplorable," said Carter, "but it does not license transphobia. Trans people receive this abuse (and far worse) every day - even more in the wake of Rowling's recent tweets."
GLAAD, the media watchdog formerly known as the Gay & Lesbian Alliance Against Defamation, asked brands and companies associated with Rowling to address the comments.
"J.K. Rowling proactively spreads misinformation and has refused conversations with LGBTQ leaders who merely want to have a dialogue and let her know the negative impact that these tweets have," said Anthony Ramos, GLAAD's head of celebrity talent, said in a statement sent to Variety. "A generation raised on J.K.'s own books about embracing differences is now making their voices loud and clear and if she refuses dialogue, then companies that partner with her should tell the community where they stand. Studios, networks, and brands affiliated with J.K. Rowling owe it to their transgender employees and consumers to speak out against her inaccurate and hurtful comments."
Rowling's tweets started Saturday with a comment on an article titled, 'Creating a more equal post-COVID-19 world for people who menstruate.'
"'People who menstruate.' I'm sure there used to be a word for those people. Someone help me out. Wumben? Wimpund? Woomud?" she wrote. This was labeled transphobic and replies accused Rowling of being a Trans-Exclusionary Radical Feminist (TERF).
Last December, Rowling also received backlash for supporting Maya Forstater, a researcher who lost her job after tweeting that a person can't change their biological sex.
Daniel Radcliffe, Emma Watson and Eddie Redmayne are among the cast members of Harry Potter and Fantastic Beasts and Where to Find Them, movie franchises based on Rowling's books, who've publicly addressed Rowling's comments.
"Transgender women are women," Radcliffe wrote Monday in a blog post. "Any statement to the contrary erases the identity and dignity of transgender people and goes against all advice given by professional health care associations who have far more expertise on this subject matter than either Jo or I."
In a Tuesday statement to Variety, Redmayne said: "Respect for transgender people remains a cultural imperative and over the years I have been trying to constantly educate myself."
Watson tweeted Wednesday, "Trans people are who they say they are and deserve to live their lives without being constantly questioned or told they aren't who they say they are."