“You didn’t like when I stopped you at Twiggs.”

“It startled me,” he said, not meeting her eyes. He cleared his throat. “I get deep in my head sometimes. You may have noticed that I can be a bit…” He unleashed the smile again, but only briefly. There and then gone. “Intense.”

“I’ve spotted the trait once or twice.”

Deftly, he unscrewed the last vial. “So: origin story. While I was in graduate school, there was a woman in David’s lab named Rhea.”

A woman, Jess thought. Of course.

“We were rivals, in a way.”

The way he added the last three words to the sentence clearly communicated Rivals who also fucked.

River pulled the needle out and immediately covered the puncture site with a square of gauze. He held it there firmly with his thumb, the rest of his hand lightly curled around her arm. “One night, at a party at someone’s house,” he said, “we started talking about the Human Genome Project from the nineties.”

“As you do at a party.”

He laughed, and the full, genuine sound delivered an erotic shock like a spanking. “Yes. As you do. We were talking about the implications of knowing every gene, the way that information could be manipulated. Could you, for example, screen people for certain jobs based on their genetic profile?”

“How very Brave New World.”

“Right?” He checked beneath the gauze to see if she was bleeding and, satisfied, reached for a fresh square, fastening it to her arm with some medical tape. “Anyway, I guess the drinks flowed and eventually I brought up whether it was possible to identify sexual attraction through DNA. Rhea laughed and said it was the stupidest thing she’d ever heard.”

Jess stared at him, waiting for the rest of it, and the heated effect of his laugh slowly faded. “That’s it?”

“I mean, that’s not it it,” he said, grinning shyly. “It turned into a real scientific undertaking, but if you’re wondering whether the project was sparked in a moment when a woman mocked me, you wouldn’t be entirely wrong. But it isn’t supervillain levels of insecurity or vanity; it was a genuine curiosity at first. Like a bet. Why did she think it would be possible to profile someone for an engineering job versus a graphic design position, but not for relationships? Aren’t both ultimately about suitedness and gratification?”

He had a point.

His face tipped down, he laughed quietly as he checked the labels. “Anyway, Rhea wasn’t the last person to mock the idea.”

“What does that mean?”

“Imagine being a fairly well-respected young geneticist and word gets out that you’re planning to use your expertise to find who’ll fall in love with whom.”

“People were dicks about it?”

He tilted his head side to side, a yes-no. “Scientists are often pretty critical of other scientists and what we choose to do with our time and knowledge.”

“Sounds like the literary world and Fizzy.”

His brows went up. “Oh yeah? How so?”