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Pheromones come in two forms: "primer pheromones" that cause slow, long-term physiological changes such, as hormonal effects; and "releaser" (or signaling) pheromones that produce rapid behavioral effects, such as mating. Pheromones are not cognitively processed chemical signals, but rather elicit instinctive and presumably unconscious responses. Moreover, pheromones typically do not have a scent per se and are often in a liquid medium. As such, in mammals, including primates, which use pheromonal communication they are processed through a non-scent organ system, the VNO and accessory olfactory bulb. We don't have this physiological hardware and in my last posting I left you hanging with the question of whether we can respond to and use pheromonal communication. These are two possible answers: (1) No or (2) Yes. And if the answer is "Yes", how do we do it?
The best demonstration of a human pheromone is the "McClintock Effect": the coincidence that women who live together seem to get their periods at the same time of the month. It is known as the McClintock Effect after Martha McClintock's observation that girls in the same dormitory began cycling in synchrony over a college semester. Since her first observations numerous experiments have demonstrated that applying the sweat from one woman to the skin above the upper lip (i.e., under the nose) of other women is capable of altering the others' menstrual cycles to be in synch with the donor woman1. This is an example of a primer pheromone—a physiological change that occurs over time. There is good evidence for the McClintock Effect as many experiments have replicated the finding, but it has also been criticized as simply due to statistical artifact. The hormonal changes that take place over the approximately 28 days of the human menstrual cycle are graphically depicted as a wave. It is mathematically the case that waves with different cycles will come into synch with each other over time and then drift apart2. Therefore, the menstrual synchrony effect could be nothing more than the product of mathematics and the fact that it is much more interesting to notice a coincidence than the lack of one. Nevertheless, if we accept the McClintock Effect as a true human pheromone, how do women respond to it if we are missing the organ-system for detection?
One explanation is that the chemicals responsible for inducing menstrual synchrony are transmitted through the skin. The sweat from one woman is absorbed through the skin (e.g., above the lip) of the woman who receives it and over time enters her bloodstream causing changes to her hormonal system. In a naturalistic setting this could be quite plausible too. Women who live together touch the same items and each other all the time—borrowing clothing, picking up a book or mug, and touching hands to exchange items—and therefore are likely swapping sweat on a regular basis. An alternative explanation is that the chemicals responsible for the McClintock Effect are actually processed through the olfactory system, and that in both experimental and naturalistic settings smelling the donor woman's sweat is the key to changing the other women's menstrual cycles.
The main compounds that give our underarm odor its bouquet are carbon chain acids, but there are other chemicals present too, in particular one that has recently received a lot of press—androstadienone. Androstadienone is a steroid and a derivative of testosterone. It is also found in higher levels in male than in female sweat. These features make it an intriguing candidate as a possible human sex pheromone.
Two recent studies have provided suggestive evidence that this may be the case. One study tested whether there were changes to a woman's mood when she was exposed to androstadienone or a control solution in the presence of a male or female experimenter. When the female experimenter was present, androstadienone had no effect, but in the presence of the male experimenter androstadienone increased women's positive mood and decreased negative mood factors3. The control solution had no impact on mood regardless of the experimenter. Another study with only a male experimenter found that sniffing androstadienone increased women's self-rated mood, sexual and physiological arousal, and even affected secretion of the hormone, cortisol4. These results have led to the speculation that androstadienone is a "modulator" pheromone for women in certain social contexts-the presence of men. However, it is possible that the specific male experimenters in these studies had an influence on the women's responses, and until more research is done on the "male context effect" conclusions remain tentative. Furthermore, the level of androstadienone that women were exposed to in these studies was a million times higher than the amount a normal male emits. Thus, the ecological validity of androstadienone as a naturally active human pheromone is questionable. Nevertheless, it is tempting to contemplate that high levels of androstadienone worn by men could sexually modulate women's mood and physiology.
Men, it seems, may also be affected by sweat based chemicals. In a recent experiment, Geoffrey Miller and his colleagues found that professional exotic lap dancers earned almost twice as much in tips (averaging $335/night) when they performed during the ovulatory phase of their cycle compared to the menstrual phase of their cycle (averaging $185/night)5. But the dancers all claimed that they performed the same way every day and that their behavior to the patrons was always consistent. So were the men responding to some chemical sex signal emitted by the dancers when they were fertile?
There were no independent assessments of the dancers performances over their cycles, and therefore it is not known whether they truly did perform without variation from day to day. Female libido is known to be higher during ovulation, as is moodiness and physical discomfort during menstruation. Therefore, the dancers may not have realized that they behaved more sensually during fertile days and less so while menstruating. Moreover, what the male patrons were responding to-the dancers' scent, looks, moves or demeanor—is not known. So the cause of this provocative finding is still a mystery.
Much more work awaits to be done on men and women as recipients of each other's chemical signals and so for now I leave you to decide. Will the aromachemical industry find and bottle a sure-fire human pheromone for sexual arousal? Or will we need to rely on how our conscious noses respond to each of our unique and fragrant bodies to tell us whom we desire?
1. McClintock, M.K. (1971). Menstrual synchrony and suppression. Nature, 229, 244-245.
2. Wilson, H. C. (1992). A critical review of menstrual synchrony research. Psychoneuroendocrinology, 17, 565-591.
3. Lundstrom, J.N. & Olsson, M.J., (2005). Subthreshold amounts of a social odorant affect mood, but not behavior, in heterosexual women when tested by a male, but not a female experimenter. Biological Psychology, 60, 197-204.
4. Wyert, C. et al. (2007). Smelling a single component of male swat alters levels of cortisol in women. The Journal of Neuroscience, 27, 1261-1265.
5. Miller, G. et al. (2007). Ovulatory cycle effects on tip earnings by lap dancers: economic evidence for human estrus? Evolution and Human Behavior, 28, 375-381.
Rachel Herz is the author of The Scent of Desire and on the faculty at Brown University.
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