With Valentine’s Day fast approaching, don’t be surprised if coupled people suddenly seem more hostile. New research presented at the annual meeting of the Society for Personality and Social Psychology shows such behavior is actually rooted in fear.

In three studies with 130 people in long-term relationships, Jon Maner, a psychologist at Florida State University, and his colleagues discovered that those people felt the need to lash out at potential threats in order to protect their own commitment to their partner.

Basically, thinking about “intense sexual desire” for a partner didn’t elicit feelings of hostility or jealousy about outsiders — but thinking about “intense love” for a partner did. And attractive third-parties were perceived as the most threatening, especially if they were in close proximity.

To wit: in one of the experiments, student volunteers were told that researchers needed their help evaluating prospective daters for a new university dating site. The students were then shown a number of profiles of “attractive, interesting, outgoing, fun-loving” people of their own sex who lived nearby, with details that were designed to be as threatening as possible.

Students who were reminded of their deep, romantic love for their partner responded harshly to the potential daters, calling them “unattractive,” “unfriendly” and other derogatory adjectives. Dr. Maner to concluded, “Love, arguably the most positive of all human emotions, also comes with a dark side.”

Jennifer Leo, a study researcher and graduate student at Florida State, agreed, adding, “Ultimately, love works in the service of protecting the relationship and maintaining it into the long term. Even if that means acting out.”

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