“Our culture  or our scientific culture, anyway  is leaning prettily heavily right now toward the wired theory,” Mr. Bergner said. “The idea that someday not so far in the future we will be able to take detailed enough images of the brain to determine where the anatomical difference is located.”

But, he added, “I’m not quite ready to go there.” He then described a weekend he spent at a Connecticut motel observing a mixer between female amputees and their male devotees, as men who are attracted to such women are known. “After a whole weekend your vision ever so slightly starts to shift,” he said. “Not that I became an amputee devotee; far from it. But I began to see as these other men were seeing. That’s what our culture does, and it has a tremendous effect. We find attractive what others do, and I don’t think all the M.R.I.’s in the world are ever going to get to that.”

The other big thing he has learned, Mr. Bergner said, is that the lines defining what is normal sexually and what is not are vague at best. We abhor pedophilia, for example, and yet our culture worships teenage girls. “When you get to the Baroness, the blurry lines are barely lines at all,” he said. “When you start talking about the connection between pleasure and pain, there’s a whole body of psychological literature that says that’s pretty common. To watch the things the Baroness does and the people who submit to her is to be attracted  at least abstractly.

“The whole language of S&M is appealing: what we want from sex is that experience of being taken somewhere deeper into ourselves and to a deeper connection with someone else. That’s what the Baroness is offering, perhaps at a dangerous price.”

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Mr. Bergner’s previous books were about the Angola prison in Louisiana and the civil war in Sierra Leone, and both demanded arduous and often uncomfortable reporting about worlds most people ignore. He said that reporting “The Other Side of Desire” didn’t seem all that different.

“I decided I was going to go inside eros, inside our sexual selves,” he explained. “Not to be melodramatic, but that sounds like a pretty extreme journey. When I went to Angola, I thought, ‘Here’s a chance to look at why we live.’ There were people living lives there that were all but exterminated, and yet those lives were in some ways pretty full. In Sierra Leone I thought I could learn something about myself, something about race. I thought that from all these people I’d learn something about us as human beings and the force of the erotic, and when I did this book it didn’t feel like a departure, really, just another extreme.”