Half a century after the discovery of pheromones in animals, scientists have yet to conclusively identify a single such chemical in humans. Yet the term is bandied about regularly in reference to people and the supposedly silent means by which they communicate.
Pheromones will improve your sex life, a common sales pitch goes.
For certain, animals use pheromones to communicate nonverbally, transmitting the chemical signals often through air. The purpose is often related to mating or defense of territory.
Peter Karlson and Martin Lüscher first proposed the word "pheromone" in 1959, referring to a chemical cocktail emitted by an animal and detected and responded to by other creatures of the same species. That same year, researchers reported the identification of the first pheromone (called bombykol) in silk moths.
Since then, such chemical equivalents of text messages have been reported in various animals, including some mammals, writes Tristram Wyatt of the University of Oxford in an essay published in the Jan. 15 issue of the journal Nature.
However, the hunt for a human pheromone has come up short.
"We can demonstrate the effects of what are putatively pheromones, but we haven't been able to pin down the chemical identity and show this particular compound or small set of compounds are responsible for outcome A, B and C," said Charles Wysocki, a behavioral neuroscientist at Monell Chemical Senses Center in Philadelphia who was not involved in Wyatt's review essay.
That apparently hasn't stopped the press and private entities from sounding the horn.
"If you go onto the Web and put in 'pheromone' into Google, you'll get something like half a million hits, most of them trying to sell you something that will make you irresistible," Wyatt told LiveScience. "They're basically trying to sell you a 'releaser' pheromone, but none has ever been identified."
Releaser pheromones trigger a behavioral response (such as wooing a mate), while so-called primer pheromones cause physiological changes.
Scientists have observed what they think are the effects of human primer pheromones, including studies showing that some compound in the extract from a woman's armpit can cause menstrual cycles of nearby women to sync up. And a recent study found that women can smell a guy's sexual intentions.
Nursing infants have been found to turn toward a lactating mother's breast, suggesting some scent molecules drive the response.
But without any actual chemicals identified as pheromones, scientists can't test effects on humans, so the jury is out as to whether we communicate via pheromones.
"As far as releasers, it may be that we simply don't have them," Wyatt said. "Certainly courtship and everything else is so complex in humans that it may be that the things that are really important are visual and social signals."
Recent research showed that at about the same time our primate ancestors gained color vision, they also lost the genes for so-called vomeronasal organ (VNO) receptors, Wyatt said. Non-human animals use the organ to detect pheromones. (Turns out, mice use both their VNO and main smelling system to detect pheromones, so maybe humans don't need that specialized organ.)
"It may be at that point that we moved from running things mostly by pheromones to doing things much more in the visual fashion," Wyatt said.