The article: Automatic inattention to attractive alternatives: the evolved psychology of relationship maintenance

There can be important reproductive benefits to maintaining a long-term romantic relationship. As a result, humans may possess evolved psychological mechanisms designed to help them maintain their commitment to a long-term mate, particularly when faced with attractive alternative relationship partners. The current study identifies a relationship maintenance process that involves being inattentive to alternative relationship partners. Experimentally eliciting thoughts and feelings of romantic love—an emotion thought to have evolved for the purpose of relationship maintenance—reduced attention to alternative partners at an early, automatic stage of visual perception. Consistent with evolutionary models of mate selection, this reduction in attention was observed only for opposite sex targets displaying high levels of physical attractiveness. This research illustrates the utility of integrating evolutionary models of mating with theory and method from cognitive science.

The experiment:

LOVE can make you blind, quite literally.

Jon Maner at Florida State University in Tallahassee and colleagues asked 57 students in heterosexual relationships to write about occasions when they felt extreme love towards their partner. Another 56 students wrote about feeling extreme happiness.

The students then viewed 500-microsecond flashes of 60 photos, comprising equal numbers of highly attractive and average-looking men and women. As the faces disappeared, they had to rapidly identify shapes that appeared on the screen - a measure of their subconscious visual attention to the photos.

Students primed with thoughts of love took significantly less time to identify shapes after viewing an attractive face of the opposite sex, compared with those who had written essays on happiness. It was as if being in love meant that they were "repelled, rather than grabbed", by attractive faces, says Maner. This may help explain why people in love do not seek out other mates.

The religious analog:

But I say to you, anyone who stares at a woman with lust for her has already committed adultery with her in his heart. (The Gospel of Matthew 5:28)

What the new research would seem to indicate is that the latter is less a mystical sacred sexual breach than a folk observation of the same phenomenon confirmed by academic experiment--attention to another is arguably evidence of a decline in commitment. Important to remember in this context: adultery back in the day was less about sex and more about one's contractual commitment to provide material resources.