Havlíček's latest study, for example, found that contrary to accepted belief, a woman's odor changes perceptibly during her monthly cycle. "In species like chimps and baboons, ovulation is advertised, while in humans it was believed to be concealed," he says. "This study found that it's not concealed. It's not apparent, but it is perceivable."

How exactly did Havlíček figure that out? He rounded up 54 volunteers — 12 women and 42 men — all students aged 20–30, all working as unpaid volunteers. The women wore cotton pads in their armpits every day for five weeks, and the men smelled them.

"Each pad was worn for 24 hours," explains Havlíček. "The women couldn't wear deodorant or perfume, couldn't eat onions, garlic or spicy foods, and couldn't drink alcohol or smoke."

The men then had to smell each pad and rate it. "We used a seven-point scale with ratings for intensity, femininity/masculinity, pleasantness and attractiveness," says Havlíček. The men were also encouraged to write their own descriptions and thoughts about what they smelled. Reactions were mixed.

"Some men were like, 'Who is No. 6 and can I meet her?' " Havlíček says. "My response was, 'We're not a lonely hearts club.' Others were like, 'What is that dirty odor I'm smelling?' So there was a wide reaction between smellers."

Havlíček found that reactions to an individual woman's odor tended to stay the same, no matter how much it changed over the course of her cycle. "If you like someone's smell, you'll like it all the time, just sometimes slightly more or slightly less," he says. "Ratings between people were varied, but men changed their ratings only slightly per person."

Lukáš Kratochvíl, a Prague resident and colleague of Havlíček's, was one of the volunteer sniffers. Even though he participated in the study nearly four years ago, some things you just don't forget. "Scents of some of the samples were quite pleasant and interesting, if sweaty cotton can smell interesting," Kratochvil recalls. "But others were absolutely awful, disgusting."

So now we know how. The next question is, why?

Havlíček did his undergraduate studies in biology and earned his master's degree in physical anthropology. Over the course of those studies, he became interested in ethnology and evolutionary biology. When he began his PhD, he knew he wanted to focus his research on behavioral studies — specifically, men's choices when it comes to women. But his focus on smells came slowly and unconsciously.

"I knew I was interested in behavioral studies, and a lot had been done on face and body attractiveness, but not much on odors," he says. "The idea developed in chatting with female friends; women are much more attuned to smells."

There was also the lure of having a field to himself. "No one in the Czech Republic was doing this research, and the methods are still not well-established," he says.

What does the future smell like for Havlíček's research? "I'm definitely interested in continuing," he says. Questions he would like answers to include: Is a person's primary partner more sensitive to changes in body odors? In some women, the odor changes were quite obvious, while in other women, they were not. Why is that?

"There are so many influences," notes Havlíček. "In everyday life, we use deodorant, perfumes, eat garlic and spicy foods. Are the smells still perceivable when controls aren't set?"

Despite his fascination with odors, Havlíček realizes that it's only part of a much larger picture. "We perceive each other not in such artificial conditions [face, odor] but holistically," he says.