In the mean time they were exposed to one of three olfactory stimuli: androstadienone, estratetraenol, or a control solution, all of which smelled like cloves and were perceptually indiscriminable.

The results revealed that smelling androstadienone systematically biased heterosexual females, but not males, toward perceiving walkers as more masculine. By contrast, smelling estratetraenol systematically biased heterosexual males, but not females, toward perceiving walkers as more feminine, Zhou said.

Interestingly, homosexual males exhibited a response pattern akin to that of heterosexual females, whereas bisexual or homosexual females fell in between heterosexual males and females, she said.

While the visual gender cues were extremely ambiguous, smelling androstadienone versus estratetraenol produced about an eight percent change in gender perception, a statistically very significant effect, Zhou noted.

"The results provide the first direct evidence that the two human steroids communicate opposite gender information that is differentially effective to the two sex groups based on their sexual orientation," the researchers wrote in their paper. " Moreover, they demonstrate that human visual gender perception draws on subconscious chemosensory biological cues, an effect that has been hitherto unsuspected."

The findings were published in the U.S. journal Current Biology.