In 1994, John Tierney followed Helen Fisher around the American Museum of Natural History, hoping she could put the Manhattan singles scene into perspective for him. She came through solidly.

“I’ve been quoting her since then, for decades,” he said. “Helen always has something smart and quotable to say on love.”

Dr. Fisher, an author, anthropologist and human behavior researcher, is among the nation’s most cited love experts. Mr. Tierney is an author and former New York Times columnist.

At the time of the museum meet-up, love was a strictly professional pursuit. Dr. Fisher, single since a four-month marriage in the late 1960s, was in a long-term relationship. Mr. Tierney was settled in his second marriage after a divorce.

The two maintained a cordial relationship, trading questions and answers on romance, until the summer of 2014, when a mutual friend, Gerry Ohrstrom, invited them on a group getaway to a ranch in Montana.

“We spent a lot of time laughing that week,” said Dr. Fisher, 75. Between hiking, fishing and horseback riding, it emerged that both were single. Mr. Tierney, 67, was a few months into what would become a lengthy divorce.

At the end of the trip, Mr. Tierney drove Dr. Fisher to the airport for her flight to New York. As “Maria Stuarda,” the opera they listened to on the car radio, wound down, he made a declaration. “All of a sudden he says, ‘I don’t think I’m ever going to get involved with a woman again.’ I said to myself, ‘Well, I’m the only one in the car. He’s probably trying to tell me something.’”

Both had felt the stirrings of attraction at the ranch.

“When you start falling in love, your brain releases the chemical dopamine, which activates your neural receptors and makes you feel both pleasure and a euphoric sense of purpose,” Dr. Fisher said. But for Mr. Tierney, the timing of the chemical flood was off. “I had just become a single father,” he said. “I didn’t have the bandwidth for it.”

Dr. Fisher appreciated his honesty. For the next year, they saw each other sporadically on New York opera and restaurant outings with friends. “At the end of the evening he’d give me a hug,” she said.

Then, in May 2015, came a breakthrough. Before a game of pool at Amsterdam Billiards, they stopped for dinner at Porteno in Chelsea. Dr. Fisher pulled a cocktail napkin out from under her glass at the restaurant and said, “Why don’t we write down what we’d secretly like if we win at pool?” She wrote “a real kiss.” He wrote “sex and clarity.” He won.

As he walked her home that night, she said, “I do study love. When you start to make love with someone, you contribute to the brain circuitry for attachment. Are you willing to take that chance?” Mr. Tierney said he was. By November, though, he was worried their relationship was getting too serious. He broke up with her in Grand Central Terminal.

“I was just numb,” she said. “I cried.”

But then, on New Year’s Eve, came an email from Mr. Tierney. He told Dr. Fisher she was the best thing that had happened to him that year. Two weeks later they were dating again, without a commitment, until his son, Luke, left for college in the fall of 2017. Then they worked out an arrangement that still holds.

“I said, I’ll marry you, but I’m not moving in,” she said. She maintains her apartment near Central Park, and he lives in the Spuyten Duyvil neighborhood of the Bronx, where he proposed on July 2. Two nights a week, they say good night over the phone. “I miss him terribly, but it’s a great way to have a really long-term romance.”

On July 21, at the ranch near Red Lodge, Mont., where both first experienced the mutual dopamine rush, they eloped. Mr. Ohrstrom, a New York investor and philanthropist, officiated wearing a king of hearts costume.

Dr. Fisher said her relationship with Mr. Tierney is one she has enjoyed “living, not dissecting.” And Mr. Tierney knows he’s in good hands, relationship-wise. “At first I did wonder about the wisdom of dating the world’s most-quoted authority on love, but I quickly discovered it was a blessing,” he said. “Helen doesn’t play games. She knows what she’s doing.”