Pheromones are probably not why people find you attractive

Are you really sending out airborne aphrodisiacs to potential mates?

In 2010, dozens of people gathered in a Brooklyn art gallery to smell each other’s unlaundered t-shirts. This was no fetish party, but an attempt at seduction through smell. The premise of the “Pheromone Party” was simple: unzip one of the many numbered bags containing someone else’s previously worn t-shirt and take a sniff. If you like what you smell, a date with the owner could be on the cards.

This olfactory outing was a success, and subsequent Pheromone Parties have now been held in Los Angeles and London. According to one magazine, they are for people “hoping to find love using their sense of smell and faith in science”.

The idea that human pheromones influence our chances of sex and love permeates pop culture. It means some people are even willing to buy perfumes called ‘love potions’. Some contain androstenone – the most potent of all sexual attractants – which its sellers claim increases the libido of women while simultaneously making men more attractive. Androstenol, meanwhile, is promised to make you seem more approachable to the opposite sex. Other pheromones are sold as “icebreakers”, masculinity boosters, and chemical comforters during sexual intimacy.

So, does the science actually show that pheromones transform our prospects of sex? Is love literally in the air, or do humans just smell?

The term pheromone was coined in 1959. Peter Karlson and Martin Luscher, two researchers from the Max Planck Institute for Biochemistry in Munich, Germany, argued that within the cloud of molecules produced by animals some were more special than others. They were similar to hormones within the blood, but emitted away from the body, influencing the behaviour and physiology of others. Rather than just odours – a broad term for molecules that can elicit a spectrum of responses – pheromones were shaped over evolutionary time between members of the same species for a precise purpose.

That same year, the first pheromone was discovered. Produced by female silkworm moths (Bombyx mori), the molecule Bombykol attracts the attention of males from miles around. It is a sex pheromone, an airborne aphrodisiac. Bombykol fit the definition of a pheromone perfectly. It was simple, just a single molecule. It was specific, working only on that moth species. And it elicited a stereotyped response; time and time again males would flutter to the source of this molecule even when there was no female to be seen.

Excitement was in the air. Hormones were old news, and pheromones were the new thing. Shortly after this doublet of studies, one author wrote: “Endocrinology has flowered magnificently in the last 40 years; exocrinology is now about to blossom.”

And so it did. Pheromones started to speckle and then spread through the scientific literature. On every branch of the tree of life, from microbes to mice, these chemical cues seemed to be influencing behaviour and physiology of others. Pheromones floated through the air, swirled in the oceans, and adhered to the ground. Some were even passed along directly, from individual to individual, like personal molecular messages.

Sex pheromones were just the beginning. Each discovery brought another distinctive purpose into the fold, from reshaping the reproductive cycles of prospective mates to acting as memory devices.   

And, in 1971, humans were brought into the mix for the first time.   

It began with a finding that has now become common wisdom: when in close proximity, the menstrual cycles of women converge, like synchronising body clocks. The idea is founded and backed by a famous study from Martha McClintock, then a student at Wellesley College in Massachusetts, published in the journal Nature. In a sample of 135 female students sharing a suburban dormitory for six months, menses became closer and closer together as time went on. Ever since, this heavily cited study has been used as evidence for the existence of human pheromones, priming females to the same period. 

“Every storyteller wants it to be true,” says Tristram Wyatt from the University of Oxford. “But analysed in more detail, it probably isn’t happening.” A plethora of studies have failed to replicate McClintock’s original findings, and after poring over the data, one of her post-doctoral researchers found that it might be a statistical artifact. In other words, this phenomenon is just as likely to occur by chance as through chemical communication. The frequency, length (five days out of 28), and variability of menstruation in women make synchronisation very likely.

Back in the 70s, however, the apparent discovery was the source of much excitement. Off the back of McClintock’s study, the British physician Alex Comfort published a commentary titled the Likelihood of Human Pheromones, stating that it would not be long until the first human pheromone was discovered.

That same year, H A Cook, writing in New Scientist, purported to have found at least two examples in human breath and sweat. One, as far as he could judge, had the same odour of garlic. “This occurs in human female breath when the female is sexually aroused, and is not caused by the eating of garlic,” he wrote at the time. Cook went on to suggest that this pheromone aroused males, and was the reason women use small amounts of garlic in cooking. “The second human pheromone is the smell of fear, which is certainly apparent to dogs,” he continued, not taking into account the species-specificity of pheromones.  

Such claims were easily discounted and ignored. But others stuck. Follow the trail of supposed sex pheromones in humans and you arrive at a conference held in Paris in 1991.

Earlier that year, Linda Buck and Richard Axel, two biochemists from Columbia University in New York, had discovered a family of olfactory receptors in mice, each encoded by a single gene. Within a mammal’s nose, each receptor binds to a specific odorants, initiating a cascade of signals along neurons in to the brain. Like an olfactory barcode, different ratios and amounts are translated into different smells. “This was the first time we could see how we smell,” says Wyatt. In 2004, the two researchers were awarded the Nobel Prize for their work.

And yet, at the 1991 conference, another study stood out. A couple of psychiatrists from the University of Utah claimed to have found two sex pheromones in humans. “They came along and they made this big splash,” recalls Richard Doty, director of the University of Pennsylvania’s Smell and Taste Center in Philadelphia. “But everybody thought it was a little strange.” In 49 volunteers, their ‘putative human pheromones’ were said to increase the electrical activity between the vomeronasal organ (VNO) and the brain. In mice, and many other mammals, the VNO, or “secondary nose”, detects a cocktail of chemical cues independently of the primary olfactory system.

A study from 2000, written by McClintock, supported their findings and pushed the use of human pheromones into vogue.

However, there is no evidence that old world monkeys and great apes, including humans, have the ability to detect such chemical cues in the same way as mice. We simply don’t possess a functional VNO. Not only is the organ reduced to a few pores inside our noses, the nerves between these structures and the brain simply don’t exist in these primates. It is a vestigial organ, a hopeless hangover from our deep mammalian ancestry.

“There’s nothing like that in humans,” says George Preti, from Monell Chemical Senses Center in Philadelphia. And yet, the same putative pheromones are still sold today.

In fact, some researchers, including Doty, believe that no mammals are strongly influenced by pheromones. “There’s no evidence that there are signals controlling creatures the way we’d like to think,” he says. This viewpoint reached its apotheosis in the 1970s, with Doty commonly at the vanguard. Mammals, many argued, were far too complex to be at the whims of such simple molecules. As one author wrote in 1976, “One wonders at this point whether the pheromone concept, so useful in insect behaviour and physiology, should be bastardised to the point where it is used to cover situations in mammalian behaviour.”

Take copulin, a collection of molecules discovered in the late 1960s in laboratory-kept rhesus monkeys. Secreted from the vagina of females, a waft of this supposed pheromone could beget sexual arousal, masturbation, and mounting from males. That much is true. But they weren’t pheromones. The handful of males used in the study had met the females before, learnt their distinctive smell, and were simply getting turned on.

Copulin was more akin to a perfume than a pheromone. (Which is apt – copulin is also used as an ingredient in the perfume industry.)

This example isn’t alone, says Doty, and there are many examples of the term’s misuse in the literature. “People just throw the term [pheromone] around, so it’s lost its scientific validity,” he says.  Many so-called pheromones contain so many chemicals that they defy the definition of simplicity. Others only work on certain test subjects, and not others. And, just like copulin, the majority can be explained by prior learning to distinctive odour cues.

A few years ago, Jane Hurst from the University of Liverpool agreed with Doty. Mammals, and humans, were simply too complex for pheromones, she thought. Then, in 2010, she discovered darcin, a single protein found in the urine of male mice. Named after the heart-throb from Pride and Prejudice, this protein not only attracts the attention of females, but also acts as a memory prompt, allowing the female to remember the male’s other distinctive odour print and where she came across it. It’s both an attractant and a learning device. Without it – sans just one signalling protein – the female just doesn’t have an interest in, or memory of, the opposite sex.

“I have changed my mind,” Hurst says. “There are specific chemicals that have the right to be called pheromones.”

To find darcin, Hurst and her colleagues went back to basics. Using the same formula as the original 1959 study on Bombykol, the active molecule was isolated from urine and tested again on mice, in isolation. Importantly, darcin alone elicited the same response as when using pure urine. And to remove any possibility that the mice learnt the smell of choice males, the females were reared in girl-only clubs, away from the sight, sound, or smell of male mice. Any response, therefore, was innate – a product of their genetics and normal development, and not conditioned to certain masculine cues they’ve encountered during their life.

The question is, can we do the same for humans? It seems highly unlikely. “In humans it would be pretty much impossible to do the classic isolation of a pheromone,” says Hurst. Years of learning, personal preferences, and a lack of reflexive responses can break down any carefully controlled trial. There could be any number of extraneous factors not taken into account, too many other explanations for conclusions drawn.

But that might not be true for newborns. “Compared with testing adults, babies are easier, are less influenced by culture and learning,” says Wyatt. And in 2009, a study hinted at a possible point of interest. When breastfeeding, the areolar glands that encircle a mother’s nipple swell and secrete an accessory liquid to milk. When placed under the nose of babies, a stereotyped behaviour follows. They open their mouths, stick out their tongue, and start to suck. They try to feed.

Importantly, secretions taken from unrelated mothers can elicit the same suckling response, eliminating the possibility that the babies were just accustomed to their mother’s distinctive odours.

According to Wyatt, this areolar secretion is the best hope to find the first human pheromone. If isolated and synthesised in the lab, such a chemical cue could help with babies that don’t latch on – a major cause of infantile mortality in many developing nations. In rural Ghana, for instance, just a one-hour delay in suckling after birth accounts for an estimated 22% of infantile mortality.

It would also help pheromone research as a whole. “If we can find the mammary pheromone, it will give us more confidence to look for the pheromones that might work in adulthood,” says Wyatt. “And the conclusion might be that we don’t have pheromones. It still stands that no molecules have been identified.”

At the moment, the only thing that can be said with certainty is that humans smell.

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