The human nose can sniff out gender from body secretions even though people don't think they smell anything, Chinese researchers reported Thursday.
Wen Zhou of the Chinese Academy of Sciences and her colleagues found that two compounds in human bodily fluids, androstadienone in males and estratetraenol in females, effectively signal masculinity and femininity, respectively, in manners contingent upon the recipients' gender and sexual orientation.
"Our findings provide behavior support for human chemosensory communication of gender, and argue for the existence of human sex pheromones," Zhou wrote in an email interview with Xinhua.
Earlier studies showed that androstadienone, found in male semen and armpits, can promote positive mood in females as opposed to males. Estratetraenol, first identified in female urine, has similar effects on males. But it wasn't clear whether those chemicals were truly acting as sexual cues.
In the new study, Zhou and her colleagues recruited participants of different gender and sexual orientation and examined their gender judgments of computer dots that represent a person walking, known as point-light walkers.
Each participant repetitively viewed seven point-light walkers that ranged from slightly more like a male to slightly more like a female, and pressed one of two buttons to indicate whether each was a male walker or a female walker.
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.