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?"