This year, in the run-up to the Frankfurt Book Fair, interest in psychological suspense is holding steady. Despite a huge boom in titles sharing literary DNA with Gillian Flynn’s 2012 bestseller, Gone Girl, and Paula Hawkins’s 2015 hit, The Girl on the Train, editors are still spending big money in this subgenre. Although some say they’re tired of seeing these kinds of books, many are betting that readers feel differently.

A concrete definition of the subgenre is hard to come by among editors, but, generally, these titles are distinguished from general crime novels by a focus on the frame of mind of the central characters. Peter Cannon, a senior reviews editor at PW who handles mystery and suspense titles, said these books often feature an innocent person caught up in a crime—usually a woman who is trying to figure out what happened to her and why.

With Frankfurt opening October 19, a number of the novels that have just sold in big-money deals to U.S. editors have been categorized as psychological suspense. One is a debut effort called The Chalk Man by British author Caz Tudor. Acquired by Crown, the novel is about a young man being pulled back into a mystery from his childhood. (Chalk drawings he made as a boy led him and his friends to a dead body.) Another, Beautiful Things, by Barnes & Noble Discover–winner Gin Phillips, which went to Viking, is set over the course of three hours and focuses on a mother trapped in a zoo with her young son and a crazed gunman. Both books fetched sums rumored to be in the high six figures. Though neither novel has a setup that echoes the calling cards of Gone Girl or The Girl on the Train—a whodunit constructed around a kidnapping (Gone Girl), dueling narrative threads (Gone Girl), or an unreliable and unlikable female narrator (both books)—the novels are being touted as psychological suspense.

But a third book that just sold, called The Woman in the Window, does bear a notable resemblance to The Girl on the Train. Written by William Morrow executive editor Dan Mallory under a pseudonym, the novel features an unreliable female narrator who believes she has witnessed a crime. Like The Girl on the Train’s Rachel, this heroine has a drinking problem, and part of the book’s conceit is the question of whether she can accurately recall what she claims to have seen. The Woman in the Window was bought for a rumored seven figures by Mallory’s own imprint.

Given the success of Gone Girl and The Girl on the Train, it’s unsurprising that the market has seen a surge of titles in the psychological suspense subgenre. What is surprising is how much of an impact both books are having on new work currently being produced—and bought.

“This is women’s fiction right now, until some new category comes and busts it,” said Alison Callahan, v-p and executive editor at Gallery Books and Scout Press. Callahan has experienced firsthand just how hungry readers still are for books that feature dark, twisty, narratives and, often, unreliable female narrators. Ruth Ware, a British author Callahan publishes (at Scout Press), has proven to be a major success in the States; according to Scout’s parent company, Simon & Schuster, her two books, 2015’s In a Dark, Dark Wood and the July release The Woman in Cabin 10, have a combined 700,000 copies in print. “Right now it seems that a certain book club reader wants to have their bell rung,” Callahan said.

Most editors cite Gone Girl—according to its publisher, Crown, it has sold more than nine million copies (in all formats) in the U.S. alone—as having started the current craze in psychological suspense. The success of The Girl on the Train, following Gone Girl, has not only driven more writers to try their hand at psychological suspense, it has also resulted in a wave of books being published with marketing materials comparing them to one of the two novels.

Of course, publishers have never been shy about comparing a new book to an existing bestseller. It is one of their standard marketing techniques. Consequently there has been a steady stream of recently released psychological thrillers featuring the word girl in their titles. (To cite just a few, there’s Jessica Knoll’s 2015 hit, Luckiest Girl Alive; Gilly Macmillan’s September release, The Perfect Girl; and Megan Miranda’s June release, All the Missing Girls.)

That all crime books of a certain ilk are being clumped into the “girl” category is worrisome to some in the industry. Bestselling crime author Laura Lippman said she’s concerned that the proliferation of “girl” titles will make people think all these books are the same—when they’re not.

“When people compare Gone Girl and The Girl on the Train, [even these two titles] don’t have that much in common,” Lippman said. Noting that Gone Girl is a whodunit, whereas The Girl on the Train is not, she worries about new authors whose books will enter the market touted as part of this craze, only to find that the trend has reached its saturation point. This, she feels, is one of the problems with the sometimes-reductive nature of the comparison.

One editor who specializes in crime fiction and spoke under the condition of anonymity said the industry has become too casual in its use of the psychological-suspense tag. “The term psychological suspense has been overused for financial reasons,” he said, referring to the fact that too many publishers are trying to sell new titles by either calling them psychological suspense, or directly comparing them to Gone Girl or The Girl on the Train.

The downside of this approach is that “books within the genre that are excellent are treated with a broad brush” and “any crime novel written by a woman is being dubbed psychological suspense.” Furthermore, as an editor, he admitted to being “fatigued” with these titles, having read so many that have come across the transom.

Carrie Feron, executive editor at William Morrow, has worked on a few books in the mold of The Girl on the Train and Gone Girl and said that “finding really great ones is not easy.” Like many editors, Feron said she wants something that, if it recalls these books, does so without feeling derivative. “There are only so many ways a person can be unreliable—either they’re directly lying to you, or they have a problem.”

Though some editors might be getting tired of seeing familiar tropes in the works of psychological suspense they’re reading, and others may be tiring of the subgenre altogether, readers still seem hungry for the material. The Girl on the Train dominated the bestseller list throughout the summer and remains a constant atop Publishers Weekly’s paperback bestseller list. According to its publisher, Riverhead, it has sold more than six million copies (in all formats) in the U.S. alone.

More fundamental to the sustained interest in psychological suspense going into this year’s Frankfurt Book Fair is the fact that no larger market trend has emerged to replace it. For the first time in years, the fair is occurring without the shadow of a hugely successful franchise hanging over it. From Harry Potter and the Twilight saga to the Hunger Games, the Millennium Trilogy, and Fifty Shades of Grey, almost all recent Frankfurt and London book fairs have taken place while a major series was dominating not just the bestseller lists but pop culture at large. Surveying the current landscape, the biggest book-launched phenomenon going into this Frankfurt is still The Girl on the Train, the movie adaptation of which opened in U.S. theaters this week.

Of course, many editors have thought about what Lippman mentioned—that with the surge in psychological suspense titles, the market saturation point is looming. The question is when that saturation point will be reached. A few U.S. editors said they felt the subgenre is more popular in the U.K., where, some believe, readers like darker fiction.

Jennifer Enderlin, executive v-p and publisher at St. Martin’s Press, said she has heard that, in the U.K. and Germany, “books like this get bought practically overnight.” American publishers, she felt, are being “a little more cautious” given the number of psychological suspense titles already on shelves.

There are some numbers that bear out the theory that psychological suspense is more popular among British readers. Authors such as Clare Mackintosh, Fiona Barton, and L.S. Hilton, who all write psychological suspense, are faring notably better across the pond.

Mackintosh’s two books, I Let You Go and I See You, have, according to Nielsen BookScan U.K., sold more than 280,000 copies; in the U.S., where only I Let You Go has been released (with I See You slated for February 2017), BookScan reports print sales of just over 12,000 copies.

Barton has also found success in the U.K. while struggling Stateside. Her novel The Widow, per BookScan, has sold nearly 70,000 print copies in England, compared to just over 41,000 copies in the States.

Hilton’s racy thriller, Maestra, acquired in the U.S. by Putnam for a rumored seven-figures after the 2015 London Book Fair, has also faltered in this country, while doing quite well in the U.K. According to BookScan, print sales of Maestra top 63,000 in England; in the U.S., print sales sit at just above 15,000.

Felicity Blunt, a British agent at Curtis Brown Ltd. who handled the U.K. sale of The Woman in the Window, said there’s no question that “the psychological suspense genre is huge” in her country. Noting that the level of success Gone Girl and The Girl on the Train have achieved has moved them from being a publishing success story to a global pop-culture phenomenon, she believes there is “an appetite” right now for similar books around the world.

“When you talk to scouts, psychological suspense [is the thing] that people are moving on very fast,” she said. She cited The Chalk Man, the aforementioned psychological suspense novel Crown just acquired in the U.S., as an example of a book that fits this trend. She said she heard that the novel became the “fastest-selling” title ever for its U.K. agent, Madeleine Millburn.

Joshua Kendall, v-p and editorial director at Mulholland Books, thinks the success of Gone Girl and The Girl on the Train has reawakened readers’ interest in something broader, namely “suspenseful stories centered around flawed human beings.” As a result, he believes categories other than crime fiction are being affected. “Look at literary novels like The Orphan Master’s Son and A Little Life—they’re playing with elements in the genre as well,” he said, referencing the fact that both works employ a mystery, albeit a subtle one, in their plot lines.

But armed with the knowledge that acquisitions made at (or just before) this year’s Frankfurt Book Fair won’t hit the market for another one to two years, editors are trying to gauge what material is fresh enough to appeal to readers who may soon become wary of works in the Gone Girl and The Girl on the Train mold.

“This [trend] has hung on for a lot longer than many thought it would,” Callahan said. “I’ve been saying to agents that this is the first fall I can remember, in a while, where I don’t know what I’m looking for.” This, she explained, creates a sense of both freedom and anxiety. “[We fiction editors] are all sitting here, and we’ll pounce on the same thing. To a certain degree we have to wait for [that big book] to come along, and patience in that can sometimes be trying.”