Even if you don't live in Hollywood, you've probably wondered how some people manage to stay married for decades on end. When asked how he remained faithful to his wife, the late, legendary actor/god/salad dressing entrepreneur Paul Newman is said to have quipped something of, "Why go out for hamburger when you could have a steak at home."

Awww. But seriously -- with all the potential significant others out there, why are some people never tempted to hook up with, say, their shirtless lawn-mowing neighbor? I mean, what is their secret? No really, tell us.

Well, researchers from Rutgers University have a theory: When you're in a happy relationship, you subconsciously think that people who pose a threat to your bond are less attractive than they really are.

In a new study, published in the Personality and Social Psychology Bulletin , researchers told 131 undergrads that they would each be working with a new lab partner of the opposite sex. (Cue the excitement!) Next, the students were shown a picture of said mystery partner, and then asked to examine a series of 11 images and select the one that most resembled the new lab mate.

But here's what the students didn't know: One of the 11 photos was an accurate picture of the new lab partner. The other 10 had been digitally altered -- five were tweaked to make the person look less attractive, and five were manipulated to make the person look more attractive.

The findings? When the students who had a boyfriend or girlfriend learned that the new lab partner was single (and thus, a threat to their relationship), they consistently chose the images that represented a less attractive likeness.

A second experiment found that students in relationships also tended to view the future lab partner as less attractive when they were told the person was interested in dating. This was especially true if the students were happily coupled up.

Maybe you're thinking, all those undergrads were choosing the wrong pics on purpose. Unlikely -- the students were told that if they selected the correct photograph, they'd be eligible to win $50. (So like, $10,000 in college currency.)

"These effects seem to happen outside of [their] awareness," study author Shana Cole, PhD, told Health in an email.

Cole and her fellow researchers weren't totally surprised by their findings. The students were probably exerting a type of defense mechanism that's known, in science-speak, as "devaluing temptation" -- or, in other words, thinking that something isn't nearly as appealing as it might actually be.

"There are many, many ways that people can effectively resist temptations," says Cole. "But devaluing temptations seems like a particularly good one. It basically makes it so that the temptation is no longer as strong a temptation. If people don't experience a strong attraction to another person—or to a piece of chocolate cake, or cigarette, or new pair of shoes -- they won't be tempted to give in to it."

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Cole also conceded that there could be another factor at play. It's possible these undergrads were just really into their SOs: "It could be that happy couples come to value the attributes their own partners have," she said. "And then any new person they see, they compare to their partner. So these effects could occur through multi-step processes of thinking first of one's own partner, comparing the two, and then coming to the conclusion that the person isn't as attractive as one's partner."

We think that's the explanation Paul Newman would like.