Rats, moths and butterflies are all known to send chemosignals to secure mates. Similar phenomena have been suggested but not proved in humans: Studies such as Elizabeth McClintock's work in the early 1970s—in which women living together in a dormitory were found to have synchronous menstrual cycles—indicate that a sort of sixth sense exists that allows people's bodies to communicate with one another.

But no evidence was produced, says Claire Wyart, a postdoctoral neuroscience researcher at the University of California, Berkeley, "that a single component of a complex mixture like sweat could induce a change on a hormonal level" without direct contact.

Now a new study led by Wyart, published in this week's issue of The Journal of Neuroscience does just that. In addition to determining that humans use chemosignals to attract one another, the findings could one day be used to create new therapies to correct hormone imbalances—most notably alternatives to cortisol replacement, which is used in treat maladies such as Addison's disease (in which the adrenal glands fail to pump out enough cortisol, causing muscle weakness, weight loss and low blood pressure). Cortisol replacement therapy can cause mood swings, ulcers, weight gain and osteoporosis.

Wyart and her team designed their study around androstadienone, a nonhormonal, steroidal constituent of sweat, which Wyart refers to as the molecule most studied because of its effects on psychophysiology in women. "What we decided to do was not to use a complex mixture such as sweat that we cannot control very much," she says, "but, on the other end, try to use a simple component that we know is in sweat and assess how much smelling this compound can affect the physiology of the woman."

The researchers exposed 21 subjects to 30 milligrams of androstadienone and to yeast, which is not in sweat but has a similar olfactory sensation. The participants took 20 sniffs of each in two separate trials. The researchers measured physiological vital signs like body temperature, skin conductance, ear pulse, blood pressure, respiratory function and cardiac rate throughout the experiment. They also measured mood and sexual arousal by checking the levels of cortisol, a hormone that has been associated with arousal and mood, in saliva samples.

The results: smelling the androstadienone increased positive mood, total physiological arousal and sexual arousal, which grew with longer exposure. The researchers also found a significant rise in cortisol levels from the sweat component, as compared with the yeast, beginning within 15 minutes of exposure and continuing for up to an hour.

Wyart notes it was particularly interesting that there was apparently no correlation between the intensity of the scent and magnitude of the effect. "It's something very intriguing about these molecules that you don't necessarily consciously smell the odor to actually [get] a response," she says. The study confirms that the sweat component affects hormone levels in women—somewhat like pheromones between moths—but Wyart says there is no way to determine whether the cortisol level spiked in response to a mood elevation or whether the cortisol increased first, in turn causing a mood lift. Scientists speculate, however, that the cortisol was secreted in response to a mood shift. "I would guess that it did not affect cortisol directly," says Bernard Grosser, chairman of psychiatry at the University of Utah. "So the pathway is to the psychophysiological arousal or whatever it was and then that caused an increase in cortisol."

Wyart says more research is needed on other hormones, which could include studying other bodily fluids such as urine and blood. She notes that there are other studies underway to figure out the receptors in the brain that are affected by stimuli like the odor of androstadienone, which could help determine the reason for the corresponding hormone hikes.