Being in love can prompt you to judge others of your gender more harshly, even if they haven't made a move on your partner.

SAN DIEGO - The singers who croon "Love Hurts" are right — but it's not just jilted partners and unrequited romantics who are at risk. It turns out that romantic love can also burn innocent third parties to a relationship.

People who are primed to think about how madly in love they are with a partner put down other appealing members of their own sex, and are even more aggressive toward them, compared with people who are instead encouraged to ponder sex with a significant other, according to new research presented here last week at the annual meeting of the Society for Personality and Social Psychology.

"Love, arguably the most positive of all human emotions, also comes with a dark side," study researcher Jon Maner, a psychologist at Florida State University, told reporters at the meeting.

Rating others

In a trio of studies involving 130 people in long-term relationships, Maner and his colleagues found that to protect their own commitment to their partner, people would lash out at potential threats. In the first study, researchers asked students at Florida State University in long-term relationships to write about either a time when they felt intense love for their partner or a time when they felt intense sexual desire for the person — both positive relationship-related experiences. Next, the students looked at pictures of either an attractive or an unattractive man or woman, followed by a picture of a Chinese character. They were then asked to rate the appeal of the character; since the Chinese character is neutral, this question was meant to gauge the participants' leftover feelings about the real target of the study — the pictures of the men and women.

The students also filled out questionnaires about their basic levels of jealousy, answering questions such as, "How likely are you to surprise-visit your partner to see who is with him/her?"

The results showed that jealous sorts and more laid-back types rated the characters as about equally attractive when they'd thought of intense sexual desire for their partner. But when they thought about intense love for their partner, the jealous sorts suddenly became very negative about other attractive people, rating them much less appealing.

In a second study, the researchers upped the ante. They again had people in long-term relationships reflect on their love or sexual desire for their romantic partner, or some other nonrelationship-related story. But this time, the participants were told they were going to play a computer game with a partner in another room. Whoever lost got blasted with painful, but ultimately not harmful, bursts of white noise through headphones. The winner got to pick how long and how loud those blasts would be.

The researchers then showed the participants pictures of their alleged partners, who were always attractive and the same sex as the person in the experiment. Again, high-jealousy types who were reminded of their love for their partners treated this outside person harshly, blasting their eardrums with louder and longer stints of white noise.

Hey, jealousy

At this point, the researchers wondered if low-jealousy people were somehow different than jealous types. So they created something designed to really freak people out. [7 Personality Traits That Are Bad for You]

The students were told that researchers needed their help evaluating prospective daters for a new university dating site. The students then saw a number of profiles of "attractive, interesting, outgoing, fun-loving" people of their own sex, Maner said.

These photos were designed to be as threatening as possible, said Jennifer Leo, a study researcher and graduate student at Florida State. "Not only are they very attractive, and interesting, they're on their campus, they're single and they're on the prowl for a mate," Leo said.

This time, the students who were reminded of their deep, romantic love for their partner responded harshly to the potential daters, rating them as unattractive, unfriendly and other insulting adjectives. The results held regardless of students' levels of jealousy.

"The surge of romantic love leads them to derogate these people," Maner said. "The more love they felt for their partner, the more negatively they tended to evaluate these objectively attractive members of their own sex."

In fact, the jealous types even said nasty things about the daters when they weren't reminded of their love for their partners, suggesting that the threat was so strong that love's dark side kicked in without help.

The takeaway, Leo said, is that there may not be a difference between low- and high-jealousy people. All that matters is the level of threat.

"Ultimately, love works in the service of protecting the relationship and maintaining it into the long term," Leo said. "Even if that means acting out."

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