Women and men can sniff out the opposite sex via odorless pheromones, a new study suggests.
The discovery adds another piece to the growing body of evidence that humans, much like the rest of the animal kingdom, know more from their noses than previously thought.
"We know that for animals, chemosignals are actually the most used signals to communicate, whereas with humans, we think chemosensation is not really used," said study leader Wen Zhou, a psychologist at the Chinese Academy of Sciences in Beijing.
"But based on our experiences, they are still influenced by these cues, even if they don't explicitly know it."
In a recent experiment, subjects who smelled possible pheromones from the opposite sex were more likely to interpret ambiguous human figures as that sex—even when the participants didn't know they were smelling anything.
Pheromones—chemicals that can communicate sexual information—are widespread in the animal world, and some research suggests humans use them unconsciously as well.
Zhou and colleagues used videos of points of light moving in a way that fools the eye into seeing human motion.
The videos were made by filming real people in motion-capture suits with LEDs at each joint—similar to the suits used to create Hollywood special effects.
Then the scientists mathematically manipulated the dots until the "figures" had neither a typically male nor typically female gait.
Sex Pheromones Influence Gender Choice
Twenty men and 20 women watched the video animations of these ambiguous figures, as well as ones that were more obviously male or female. While watching the videos, the subjects sniffed clove oil infused with the male steroid androstadienone, the female steroid estratetraenol, or a plain oil used as a base for many cosmetics.
Men who smelled the female pheromone were more likely to identify the androgynous walker as a woman, and even were more likely to identify more clearly male figures as female than those who just smelled clove oil.
The same results applied when women sniffed the male compound: They more frequently saw the ambiguous figures as male than the women who smelled the plain oil.
Estratetraenol had no effect on women, and androstadienone didn't affect men.
This perception difference seems to be completely unrelated to what their noses told them: A blindfolded test subject couldn't tell the difference between steroid-infused clove oil and plain oil.
"It's completely below their awareness," Zhou said. "They didn't know what they were smelling, but their behavior showed these different patterns."
Zhou presented the research in April at the annual meeting of the Association for Chemoreception Sciences.