Red dresses muddle men's minds, just ask The Matrix's Neo. In a scene from the 1999 sci-fi film, the hero is famously ambushed after becoming distracted by a woman on the street wearing a slinky red outfit. Now, a new study shows how such duds attain their sway. Men rate women wearing red clothing as being more interested in sex, hinting that humans may be conditioned to associate the color with fertility.

The pull of red is nothing new. Women have donned pinkish blush and bright lipstick for nearly 12,000 years. And, if you're lucky enough to get a Valentine's Day card, it will probably come decorated in tiny red hearts. It's an effect that likely stems from biology, says Adam Pazda, a psychologist at the University of Rochester in New York state and an author of the new study. When many primate females—from chimpanzees to types of baboons called mandrills—become fertile, their estrogen levels peak, opening up their blood vessels and turning their faces bright red. This flushed complexion seems to give males the signal that it's time to make their move.

The same could be true for humans, Pazda says. In a previous study, scientists showed that men seem to be more attracted to women clothed in red rather than in a blah color such as white. That's regardless of the cut, he adds. "It doesn't have to be a red dress or a sexy outfit," he says. "It can be a red T-shirt."

To understand why, Pazda and his colleagues conducted a simple experiment. They showed 25 men a photo of a single woman doctored to look, in different cases, like she was wearing either a red or white T-shirt. The researchers then asked the volunteers to gauge, on a scale from 1 to 9, how keen the model seemed to be on romance. In other words, the men answered the question: "Is she interested in sex?"

Men interpreted the red outfit as a signal that the woman was indeed more open to sexual advances. In fact, the guys tended to grade the woman's disposition to sex about 1 to 1.5 points higher when she was wearing a red rather than a white tee, Pazda and colleagues report online this month in the Journal of Experimental Social Psychology. That perception, in turn, explains why men's lust perks up for women in red, Pazda suggests. It's well known that males tend to inflate a woman's sexual appeal if they believe she'll be more open to a pickup line.

"I think [the study] is quite good," says Paul Eastwick, a social psychologist at Texas A&M University in College Station. "It suggests to me that humans as they exist today exhibit these somewhat odd evolutionary artifacts that haven't been applicable for some time."

The team's work is "really interesting," adds Markus Maier, a psychologist at the University of Munich in Germany. But, at this point, it's impossible to say just why men adore red so much. The effect could be evolutionary, but it could also be a cultural phenomenon—in other words, learned behavior passed down from generation to generation. To figure out which, he says, scientists would need to travel to isolated corners of the world to examine just how universal red's status as the color of love is.

But it's clear that women should beware, Pazda says. Even seemingly insignificant wardrobe choices can send out a lot of unintended signals. "Wearing red may be a double-edged sword," he says. Women "may be getting sexual attention they don't want." But, he adds, there's a lesson for men, too. It's important for gents to be aware of how their attitudes toward women can be twisted by often misleading cues. That's a lesson Neo learned the hard way.

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