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Last week, while I was eating breakfast – eggs, bacon, toast and more bacon – my girlfriend mentioned from behind her laptop that the Danish parliament had just passed the world's first "fat tax," a measure that would make fatty foods more expensive for the Danes.

"Maybe you should put a fat tax on our relationship," I replied. "The more weight I gain, the more relationship benefits you withhold."

I rubbed my belly and complained once again about the extra volume I'd acquired since we'd been together. She reassured me that it was fine, fulfilling the reason I'd complained in the first place.

Silently, though, I still wondered if all those fitness magazines were right and she would become less attracted to me and our relationship would fall apart if I became rounder than when we met. I realize those evil magazine headlines are usually talking to women, but I still harboured a small worry.

My concern spurred me to research the matter and get at the truth of how weight affects relationships. And by truth, I mean I intended to find as much proof as I could that it doesn't matter.

Robin Milhausen, who ran a study on weight and sexual satisfaction at Guelph University, couldn't give me that. Instead, her recent research turned my worry on its head, suggesting that it's the worry, not the weight, that can turn into a problem.

After surveying men and women at every point on the scale, she found zero correlation between body weight and one's sex life. The factor that mattered more was how a person felt about their shape. "Regardless of their size," she told me, "our subjects had better sexual satisfaction and sexual functioning if they had a better body image."

Dr. Milhausen didn't yet have concrete explanations for the finding, but shared a few ideas. "In order to be sexually satisfied, you have to be present and focus," she explained. "If you're thinking, 'Do I look fat from this angle?' or 'Am I jiggling?' it makes it harder to get an erection or have an orgasm."

And even if you are able to be in the moment, she says bad body image might limit you from enjoying the full range of activities with your partner. "You might not expose your body in ways that could be really pleasurable," she said, invoking an improbable yet terrifying reality for a man: "You might not allow someone to give you oral sex because they'll be down by your abdomen."

Michael Inzlicht, a social psychologist who has studied willpower at the University of Toronto in Scarborough, agrees that instead of bringing partners closer, immoderate weight-watching could have the opposite effect. "Self control is like a muscle," he said. "When you use it in one domain, you have less strength to use it in others."

Dr. Inzlicht told me of a study where researchers put a bowl of chocolate in front of subjects who thought they were still waiting for the real experiment to begin. The subjects who were told not to eat the treats performed worse at the persistence and concentration tests that followed than did the subjects who weren't given any instructions at all.

"Compromise takes work, and modifying emotional reactions to things a partner does takes work," he explained. "If you're working really hard to stay fit, you will have less self-control to regulate emotions or work on the relationship."

Jeffery Sobal of Cornell University conducted in-depth interviews with couples in their first year of marriage. His paper didn't come to any neat conclusions, but simply showed that the variety of ways we relate to each other via our weight is vast.

"I remember one guy said, 'I feel like I want to be dating my wife forever,'" Dr. Sobal told me. "He really wanted to be thin and fit out of respect and love for her. He wanted to stay attractive to her for life."

Dr. Sobal made no judgment of this man when we talked, but I wondered whether there was something flawed in thinking of one's body as a gifted statue, when sharing your life with someone means loving through change.

In his paper, one of Dr. Sobal's subjects hit on the same sentiment, while also highlighting that it's important to stay healthy even if you are, as they say, fat and happy: "I guess I think [my husband and I]are going to be together forever now. So if I want to be with him for the next 50, 60 years, I've got to start eating properly."

This long view may have been put best, however, by my friend William earlier this summer while we were at his apartment drinking wine. Forty and married six years ago, he was still concerned about his own expanding girth, but he'd also come to believe that when we look at people we love, we see something less specific than a shape.

"I think we just want to know it's them," he said. "We look for a general tone of voice and that the eyes and nose are in the right place. Or maybe there's a certain way they move their arms."

Micah Toub is the author of Growing Up Jung: Coming of Age as the Son of Two Shrinks.