Could men and women rely on smell to find potential mates? Birds do, bees do—and now scientists have some reason to think that humans do, too.



Growing evidence suggests that bodily odors carry chemical signals that affect moods and menstrual cycles, but isolating the specific compounds that elicit these effects, called pheromones, has proved difficult. Wen Zhou, a psychologist and olfaction researcher at the Chinese Academy of sciences, and her colleagues looked at two compounds found in bodily fluids that, according to earlier studies, are good candidates for human pheromones: androstadienone, associated with men, and estratetraenol, from women. The two steroids were found to elicit markedly different responses in male and female test subjects.



Neither steroid has any discernible fragrance, but it is believed that the human nose picks up these chemicals. Earlier research suggests that androstadienone boosts women’s mood and cortisol levels whereas estratetraenol enhances men’s arousal and mood in certain circumstances.



Zhou and her colleagues worked with 96 subjects, half female and half male. Half of the men and women self-identified as heterosexual and the other half as either homosexual or, in the case of female participants, bisexual. The researchers presented each subject with moving dots on a screen that simulate the outline of a walking human figure. By changing the position of the dots, Zhou and colleagues could make the figure appear more masculine, feminine or androgynous. The subjects responded by judging each figure as either a man or a woman.



After the researchers recorded how each participant labeled the figures, the subjects watched more walking figures while being exposed to a solution that smelled like cloves. This mixture contained estratetraenol, androstadienone or just the cloves scent.



Zhou and her colleagues discovered that when heterosexual subjects viewed gender-neutral walkers, being exposed to the male or female steroid biased their responses. Heterosexual men were more likely to identify the figure as female when exposed to estratetraenol and heterosexual women tended to call the walkers male in the presence of androstadienone.



Homosexual men responded to androstadienone much as heterosexual women did.Homosexual or bisexual women, by contrast, showed no bias to either steroid. Taken together, the findings suggest that humans could use chemical signals to detect an individual with romantic potential, and that these cues work in a sex- and orientation-specific way. The work was published in Current Biology on May 1. (Researchers at the Chinese Academy’s State Key Laboratory of Brain and Cognitive Sciences and the University of Minnesota also participated.)



Psychologist and olfaction researcher Johan Lundström, of the Monell Chemical Senses Center, who was not involved in this study, calls the work “the most elegant” findings yet for androstadienone and estratetraenol’s behavioral effects. “I think they are bringing the field forward,” Lundström says.



Zhou isn’t ready to declare that these two steroids are human pheromones. For one thing, their test subjects were exposed to steroids in much higher concentrations than people naturally secrete. “It is very important to examine the effects of steroids at more ecologically relevant concentrations,” Zhou says. In addition, the underlying mechanism by which people would receive and respond to these steroids remains a mystery.



The source of these compounds is also problematic. Androstadienone can be found in women as well as men and estratetraenol has only been found in pregnant women’s urine and placentas. More study is needed to investigate how sex or gender specific these signals are.



Finally, it’s possible that instead of true chemical signaling, this is simply a case of learned association, in which people become familiar with certain chemicals from men and women through repeated exposure to intimate partners. Zhou tried to control for this by repeating their procedure with isovaleric acid, a strong smelling fatty acid that men have in abundance, instead of the two steroids. Because the acid exposure did not bias subjects toward recognizing male walkers, Zhou concluded that the response is not learned. Lundström, however, is more skeptical. The lack of bias may reflect subjects’ exposure to isovaleric acid in food: It is the compound that imbues stinky cheese with the odor of dirty socks.



Although many questions remain, it is clear that even chemicals we cannot consciously detect could have a complex effect on human sexual behavior, perhaps as strong as a handsome face or “come hither” glance.