There really was a woman who photocopied her butt at a workplace in the 1980s.

Curtis Sittenfeld, 49, heard about the incident when she was a girl and filed it away. Four decades later, the Great Butt Xeroxing makes an appearance in her new short story collection, “Show Don’t Tell.”

She mentioned it one day last week when she met up with her oldest childhood friend, Anne Morriss, in Cincinnati, where they had both grown up. Ms. Sittenfeld, who lives in Minneapolis with her husband and two daughters, was back in town while on tour for her latest book. Ms. Morriss, a leadership coach in Boston, was there to celebrate her mother’s 83rd birthday.

“It happened in my mother’s real estate office,” Ms. Morriss said. “I remember processing it with you. And you had questions!”

“It’s all I think about,” Ms. Sittenfeld replied.

Why did she do it? The mysteries of human behavior, along with the mortification that often follows an ill-considered act or remark, are of special interest to Ms. Sittenfeld, who made her name 20 years ago with her debut novel, “Prep.” She’s the patron saint of women who wish the floor would open and swallow them whole.

“People will have very different reactions to my writing,” she said. “People will be like, ‘I felt so frustrated by this character — they were so neurotic or cringey, and I wanted to reach into the story and shake their shoulders.’ Or people will be like, ‘I felt like you were inside my brain.’”

The two friends lined up behind a gaggle of schoolgirls at Graeter’s Ice Cream, a local favorite. They ordered cups of mocha chip (for Ms. Sittenfeld) and chocolate chip (for Ms. Morriss) and strolled to a park, taking advantage of the unseasonably warm day.

They sat on a bench and watched a group of middle-school-age girls in Uggs and leggings who were making a video of themselves doing a TikTok dance. The girls ran to their phones to watch the recording, deleted it, and did the dance again.

Ms. Sittenfeld, who was wearing New Balance sneakers and a blue heathered sweater, and Ms. Morriss, with her Hillary Clinton bob and silk scarf, didn’t look like they had inspired the haughty queen-bee characters in “Prep.” But Ms. Morriss insisted they had been “mean girls” back in middle school.

“Were we mean girls?” Ms. Sittenfeld said. “Obviously, I am a little defensive, but in middle school I would say that we were popular more than mean.”

Then she pondered her statement, as though cross-examining her own recollections.

“Actually,” she continued, “I’m sure we were mean. I unearthed some diaries recently. I read them to my own children, and one of my kids was like, ‘You should write an essay called ‘Diary of a Bitchy Kid.’”

Cracking open another childhood trauma, Ms. Sittenfeld recalled a time in eighth grade when she and Ms. Morriss had stopped being friends for a while. The split occurred during what Ms. Sittenfeld described as her own “social downfall.”

It came about because she had committed the faux pas of skipping a friend’s slumber party. After that, she found herself exiled from her usual peer group and sitting with the student council boys at lunch. She eventually felt so isolated that she ended up leaving the Midwest for the Groton School, an elite boarding academy in Massachusetts that provided her with material for “Prep.”

“You were curious about the world in a way that the rest of us weren’t,” Ms. Morriss said.

Ms. Sittenfeld took a moment to consider this.

“Let’s be honest,” she said. “I do not think that I seemed brilliant as a child — and frankly, it’s not like I think I seem brilliant now. Sometimes I’ll encounter writers and they’re so smart, and they’ve read everything there is, and it’s almost like they have an inaccessible intelligence. I would not say that I have an inaccessible intelligence.”

‘The Messiness of Life’

In “Prep,” Ms. Sittenfeld focused on a girl who pinballs between a hunger to be noticed and a desire to disappear. In the eight books she has published since, she has mined the terrain of female self-consciousness and status anxiety across all life stages.

In “Show Don’t Tell,” the story that opens her new collection, she examines the unspoken rivalry between a pair of students, a woman and a man, at a top graduate writing program. When they meet up at a hotel bar nearly 20 years later, the woman is the author of five best-sellers and the man is the winner of prestigious literary prizes.

“He’s the kind of writer, I trust, about whom current students in the program have heated opinions,” Ms. Sittenfeld writes. “I’m the kind of writer their mothers read while recovering from knee surgery.”

But here’s the thing about American women recovering from knee surgery: They are shaping the country’s political, social and cultural debates. Pundits want to know why a majority of white women voted for Donald J. Trump. Documentaries tell cautionary tales of affluent women who fall down social media rabbit holes leading to wellness influencers promoting dubious health regimens. Ms. Sittenfeld chronicles this demographic from within, not as an impartial observer.

“I’m not an ornithologist — I’m a bird,” she said, quoting Saul Bellow. And she isn’t bothered by fancy male critics who might be inclined to dismiss the people and subject matter at the heart of her work. “If I have an opinion, I should write a 1,000-word essay,” she said. “If I want to explore the messiness of life, I should write fiction.”

For years her books have captured the concerns of a group that has lately become a cultural fixation, middle-aged women who wake up one day and realize their lives aren’t exactly what they’d planned. After reading “All Fours” by Miranda July or watching Halina Reijn’s “Babygirl,” some are having frank conversations about sex and marriage; others are simply spiraling.

Ms. Sittenfeld’s heroines seem to want more than they should while bumping up against the limiting forces of age or wilted ambition. She has explored such women in best-sellers and two works selected for Reese Witherspoon’s book club. Hollywood executives who optioned her books have suggested casting stars like Anne Hathaway and Naomi Watts.

Her two teenage daughters have made it clear that they’re not particularly impressed by her career. “They see me as kind of ridiculous,” Ms. Sittenfeld said. “My 15-year-old will sometimes be like, ‘I can’t believe you write books, you seem so apart from the world.”

It helps that she lives in Minneapolis, where her husband teaches media studies, and which feels so distant from the hothouse worlds of Brooklyn and Hollywood. “Sometimes in interviews people will say to me, ‘Do you feel a lot of pressure in writing your next book?’ And I’ll think, Who would I feel pressure from?” Ms. Sittenfeld said. “Nobody cares what I’m doing.”

Still, the older Ms. Sittenfeld gets, the clearer she feels about what she wants to do in her work.

“Are you watching ‘Somebody Somewhere’?” she asked Ms. Morriss, referring to the HBO show starring Bridget Everett as a woman who returns to her hometown in Kansas. There’s a moment in the show, Ms. Sittenfeld recalled, in which the main character and her petite sister are talking about “the pencil test.”

“You put a pencil under your breast, and if it falls out it means you have perky breasts,” Ms. Sittenfeld said. “Then Bridget Everett’s character takes a big salad dressing bottle and wedges it under her enormous boobs. That is the tone of the storytelling I want to do. It’s not the person with the pencil falling out, but the person with the salad dressing bottle staying under her boobs.”

She added, “Isn’t it so weird and undignified to be a person?”

‘So Authentic’

Shortly before 6 p.m., Ms. Sittenfeld stepped into the Mercantile Library, where she was scheduled to give a talk. The library’s executive director, John Faherty, greeted her with some praise for her new book, while noting that its depictions of marriage were a bit dark.

“I was going to call you up and say, ‘Are you OK?’” he said.

“That’s not a blurb for the paperback,” Ms. Sittenfeld replied.

She and Mr. Faherty had become close through various book talks at her hometown library over the years. “I did an event here in 2016 for ‘Eligible,’” she said, referring to her modern-day retelling of Jane Austen’s “Pride and Prejudice,” which she set in Cincinnati. “John got everyone Skyline chili.”

“I was told you can do gender reveal parties at Skyline now,” she added, referring to the restaurant chain.

“Do they say ‘boy’ with a hot dog?” Mr. Faherty asked. “I’m afraid to ask what’s for a girl.”

“The absence of a hot dog?” Ms. Sittenfeld said with a laugh.

She grabbed her phone and opened a text from her 15-year-old daughter. “We watch ‘Severance’ as a family and she was like, ‘Can I watch it by myself?’” Ms. Sittenfeld said.

“Say no and she’ll watch it anyway,” Mr. Faherty suggested.

The thrum of voices was getting louder as the crowd assembled. Ms. Sittenfeld swapped her normal New Balance sneakers for what she called her “fancy sneakers,” which were almost identical but with blue floral decals. She went to the bathroom to apply makeup — “just a little foundation,” she said.

In the main room, Ms. Sittenfeld and Mr. Faherty sat perched in front of some 225 people, an audience that included Ms. Sittenfeld’s 77-year-old mother. Ms. Sittenfeld described the sorts of questions that come up in her new book: If you eat a cup of sauerkraut with a dollop of Thousand Island dressing for lunch every day and your spouse finds that disgusting, is it his fault or yours?

The audience tittered. An older woman in a lilac sweater buried her face in her hands, giggling. When Mr. Faherty seemed on the verge of giving away a plot point, a spoiler-averse audience member shouted, “We haven’t read the book yet!” In the front row, someone knocked over a cup of wine and then got on her hands and knees to mop it up.

When Ms. Sittenfeld wrapped up her talk, readers rushed forward to ask for selfies and autographs. In Ms. Sittenfeld’s books, her characters realize over and over again that there is no escaping the embarrassment of being alive; there’s only finding somebody who will respond tenderly or, at least, with a good-natured laugh. The ache of that recognition filled the room.

Readers toted copies of “Prep” and “American Wife” that looked as if they’d been through the washing machine. One declared she had driven three hours to get there; another boasted of a book club made up of Ms. Sittenfeld’s devoted fans.

Ms. Sittenfeld’s third grade teacher, Bobbie Kuhn, sitting in the second row, said of her former student: “She’s just as authentic as she was.”

It’s the type of compliment Ms. Sittenfeld is used to receiving.

“People will be like, ‘You’re so authentic,’ which probably means you’re saying something wrong,” she said, laughing. “It’s like somebody saying you’re brave. You’re kind of like — oh no!”