When it comes to communication, humans tend to use verbal and physical messages, but we also might use our sense of smell. It’s well known that other mammals use a special set of sensory receptors to send social and sexual information with pheromones, odorless chemicals that elicit behavior in a same species receiver. A female mouse may excite a male mouse to mate with her via chemical cues, a bumblebee back from a food foray can alert others to the bounty, and a male mouse might send out an aggressor signal to other males. Humans can also send messages without consciously knowing it. But how does this subtle chemical communication work and to what extent are we subject to its whims?
What’s in the Air?
Evidence of human chemosignals came during the 1970s, when Martha McClintock, now a professor at University of Chicago at Illinois, found that women who lived together in dorms had periods that became synchronized. Odorless compounds from the armpits of women in the late follicular phase of their menstrual cycles sped up the surge of luteinizing hormones of recipient women, effectively shortening their periods so they would become more coordinated.
Although more recent work has called to question how much this could be due to chance, McClintock and her colleagues have gone on to publish related studies that support the influence of chemical signals on ovulation and reproductive behavior. In 2004, they published a study showing that compounds collected from the armpits and breast area of lactating women and their breastfeeding infants increased the sexual motivation of other women. When exposed to these compounds, women with partners experienced increased sexual desire and those without partners experienced more sexual fantasies. The authors have also found that compounds from lactating women and/or their babies were able to modulate the ovarian cycles of other women. They hypothesize that, like in other mammals, these compounds act as pheromones, regulating group reproduction.
Why would a lactating mother send out chemicals that increased sexual arousal and influenced another woman’s menses? The authors hypothesize this is an evolutionary adaptation. Coordination of pregnancy and lactation could increase the chances of survival of offspring because the presence of a breastfeeding woman indicated that there was enough food and resources around to support the energetic demands of pregnancy and lactation. In times of food abundance and security, pheromones were released to signal to do it now or never.
Scent of a Man
Chemical signals don’t just work between the same sexes. One of the most likely candidates for a human pheromone is androstadienone, a component of sweat, especially male sweat. In 2007, researchers at UC Berkeley measured levels of the hormone cortisol in women after smelling pure androstadienone and found that a whiff of this compound maintained higher levels of cortisol in women. Because cortisol regulates some aspects of mood, exposure to androstadienone can brighten mood and sharpen mental focus, but the effect of the compound depends on where the woman is in her menstrual cycle. Preovulatory women, who are at the peak of their fertility, are most affected by it.
Regular exposure to male secretions can also influence when a woman has her period, causing women with irregular cycles to become more regular. A 2003 study published in Biology and Reproduction found that secretions from a man’s armpit (where many pheromones are thought to emanate) shifted menstrual cycles toward the normal cycle length. These secretions have also been found to reduce tension and increase relaxation, furthering evidence for the behavioral effects of pheromones.
Studies have also found that pheromones can give clues to a potential mate’s genetic makeup. The first study to indicate that chemical signals play a role in attraction was conducted by Claud Wedekind over a decade ago. Forty-four men wore the same T-shirt for three days. They refrained from deodorants and scented soaps so they wouldn’t interfere with their natural smell. Women then sniffed the shirts and indicated which ones smelled the best to them. By comparing the DNA of the women and men, the researchers found that women didn’t just choose their favorite scent randomly. They preferred the scent of man whose major histocompatibility complex (MHC)—a series of genes involved in our immune system—was most different from their own.
The Right Stuff?
While it’s clear that subtle chemical cues can influence mood and reproductive cycles of those exposed to them, some researchers question whether we have the right machinery for pheromones to work as they do in other mammals.
In mice, the tiny vomeronasal organs (VNOs) are thought to be responsible for pheromone detection and brain function. Although these have been found in humans, they’re largely thought to be vestigial and inactive, having lost nerve connection with the brain.
Still, this doesn’t rule out the role of human pheromones. Rather, it may indicate a different pathway by which information reaches the brain. One plausible mechanism is that olfactory epithelium, specialized tissue inside the nasal cavity, can pick up pheromones and send messages onto the central nervous system.
However, although pheromones can trigger predictable behavior in animals, humans are much more complex. After all, humans are under volitional control, regardless of what the chemistry is telling us to do. Our noses may tell us to procreate or to find a good mate, but ultimately it’s up to our higher thought and language centers to decide whether we like what we’ve sniffed out.
Updated July 30, 2009