The Science of Sex: Pheromones

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Last week, as another Valentine’s Day came and went, I was struck by the ever-increasing number of products on sale that claim to contain human sex pheromones — skin-borne effusions that are purported to turn people on at a single sniff. Just a few years ago the Erox Corporation, with its Realm Men and Realm Women, was the only player in this field, but now a quick Web search identifies dozens of competitors. Unfortunately for scientific romantics, just as human sex pheromones have hit a public relations high point, the scientific community has turned increasingly skeptical about their existence.

Sex pheromones were first identified in insects, where they function as powerful come-hither signals, luring prospective mates from blocks away. Many non-human mammals also release sex-specific odorants, and these are detected by a special sense organ in the nose called the vomeronasal organ or VNO. The cells in the VNO of rodents possess a special suite of receptor molecules not found in the regular olfactory mucosa, and nerves run from this organ to parts of the brain that deal with sexual behavior. When a rodent’s VNO is blocked, its sex life is seriously disrupted, a bit like the effect that graduate school has on humans.

The main proponents of the idea that humans have a comparable system have been entrepreneur David Berliner (founder of the Erox Corporation) and neurophysiologist Luis Monti-Bloch of the University of Utah. In a series of well-publicized studies in the early 1990s, Monti-Bloch claimed to find a VNO in the human nose, and identified two chemicals in human sweat that triggered specific electrical activity in the VNO: one that worked on men, the other on women. Monti-Bloch and Berliner reported that these chemicals, metabolic derivates of sex hormones, induced a sense of well being in those who smelled them. These chemicals became the active ingredients in Berliner’s Realm perfumes.

Over the last twelve months, however, a slew of studies have cast doubt on the very existence of the VNO system in humans. Three groups of researchers (at the University of Texas, at the Walter Reed Medical Center in Washington, and at the Ecole Pratique des Hautes Etudes in Paris) searched for a VNO in the noses of a large number of living persons. The two U.S. groups found evidence for a VNO in only a minority of people. The French group did estimate that as many as seventy-three percent of the population might have some trace of a VNO, but when they examined histological specimens of the structure under the microscope, they found no receptor cells and no nerve fibers connecting them to the brain. This finding is consistent with earlier anatomical studies suggesting that the VNO is a vestigial structure in humans — a relic of our evolutionary past that forms during embryonic life but then usually atrophies, even in the French.

Genetic studies support this conclusion. Rodents have three groups of genes, named V1R, V2R and V3R, that code for the receptor molecules in the VNO. Last year, molecular geneticists at the Institut of Human Genetics in Montpellier, France, searched for the corresponding human genes in the V1R and V2R groups, but all they found were “pseudogenes” — non-functional relics of what used to be working genes in our long-ago evolutionary past. To be fair, I should mention that researchers at Harvard University did recently identify one gene in the V3R group that could possibly be functional in humans. Still, the weight of the genetic evidence right now suggests that the VNO system in humans lacks functional receptors.

To get an expert opinion, I spoke recently with Richard Doty, director of the Smell and Taste Center at the University of Pennsylvania Medical Center. Doty told me he was now very skeptical of the pro-VNO results of the Monti-Bloch group. He suggested that technical problems, such as a failure to prevent test odors from spreading to the regular olfactory mucosa, could account for their findings. Human sex pheromones probably don’t exist, say Doty. Even in rodents, he adds, “pheromones” are not very pheromone-like, because they don’t elicit automatic responses as they do in insects.

Psychologist Jeffrey Schank of the University of California at Davis agrees that there are problems in the methodology of studies on pheromones. Schank formerly collaborated with Martha McClintock of the University of Illinois — famous for her claim that when women live together, their menstrual cycles align. Schank criticized a much-publicized 1998 study from McClintock’s lab that supposedly demonstrated a pheromonal mechanism for this effect, noting that the 1998 study — as well as many other studies in the field — failed to properly allow for variability in the lengths of individual women’s cycles. When he incorporated this variability, the effect disappeared.

All in all, things don’t look good for human sex pheromones — although you wouldn’t know it from the marketing of these Love Potions #10. But at least with the influx of so many new products, the prices have come down: an eighty dollar bottle of Realm was once the only option, but you can now get pheromone products for as little as $7.95. A cheap-date inducer, indeed.

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