It's not hard to tell when a guy is "happy to see you."

The twinkle in his eye, his swagger, that sexy smile — all are clear signs he's in the mood.

And, at least subconsciously, a woman can also tell by the scent of his sweat, according to new research.

Scientists have long debated whether humans, like animals, use chemical signals called pheromones to communicate sexual interest to potential mates.

Problem is, the effects of pheromones are thought to be subconscious — meaning that if we do communicate using them, we sure don't know it.

It's also hard to know what these pheromones might be and how we sense them, so researchers understand little about them.

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But if human pheromones are going to be anywhere, they're going to be in sweat, right?

Denise Chen, a psychologist at Rice University in Houston, and her colleagues devised an experiment to compare how women respond to different forms of male sweat — sweat produced in everyday situations versus that produced when a man is turned on.

The researchers speculated that if humans do produce and respond to sweat pheromones, then a woman should respond to a guy's sexual sweat differently than she does to his normal sweat.

Chen and her colleagues asked 20 heterosexual guys to stop wearing deodorant and scented products for a few days.

Then they told the men to put small pads in their armpits as they watched pornographic videos and became aroused. (The researchers confirmed, using electrodes, that the images did the job.)

Later, the guys were asked to exchange those pads for fresh pads to collect the sweat they produced when they weren't aroused.

Then the researchers recruited 19 brave women to smell the men's pads while undergoing brain scans.

The investigators used functional magnetic resonance imaging (fMRI), a technique that reveals the brain regions a person is using at any given time — even if their brain activity is subconscious.

Sure enough, the women's brains responded very differently depending on which sweat they sniffed. (And no, none of them passed out.)

The sexual sweat, but not the normal sweat, activated the right orbitofrontal cortex and the right fusiform cortex, brain areas that help us recognize emotions and perceive things, respectively.

Both regions are in the right hemisphere, which is generally involved in smell, social response, and emotion.

The findings bolster the idea that humans do communicate via subconscious chemical signals, notes Chen in her study, which was published in the Dec. 31 issue of the Journal of Neuroscience.

Our sexual intentions, in other words, may be a lot clearer than we ever intended them to be.

That crush you have on your co-worker? She may already know — at least subconsciously.

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