杏吧原创

EVER had a feeling come over you that you just can鈥檛 explain? Like suddenly getting all warm and fuzzy when you meet someone for the first time, while somebody else who looks just as good leaves you cold? Or experiencing a sudden pang of fear on a plane even though you鈥檙e totally at ease with flying?

These seemingly unrelated and illogical human reactions may have a reasonable explanation after all, although one that not everyone will be happy to hear. They may be reactions to other people鈥檚 pheromones.

Pheromones are something of a sensitive subject in human biology. Though they are found across the animal world from insects to mammals, research into human pheromones has been dogged by flaky experimental designs and dubious commercial endorsements, with the result that the entire field has a whiff of the disreputable about it. 鈥淚t鈥檚 not so much that the jury is out, but that the jury has been dismissed before the trial has begun,鈥� says Mike Meredith, a neuroscientist at Florida State University in Tallahassee, who studies animal pheromones.

In recent years, though, this has begun to change. Evidence that animal pheromones don鈥檛 always work in they way we thought, backed up by a growing number of brain-imaging studies in humans, is convincing some researchers that we really do make and respond to pheromones. As a result some think it鈥檚 time to stop asking if human pheromones exist and start investigating exactly how they affect our behaviour.

The term pheromone was coined in 1959 to describe chemicals released by insects to trigger hard-wired behaviours in other members of the same species. The classic example is female moths releasing sex pheromones to attract a mate. When pheromones were discovered in mammals, their more complex behaviour meant this definition no longer fitted the bill. Researchers have been debating the meaning of the word ever since (see 鈥淲hat are pheromones?鈥�). One useful working definition is that pheromones are chemicals which send a message that, in evolutionary terms, benefits both sender and receiver.

Whatever definition you go for, pheromones are a big part of the animal world. Animals use pheromones to transmit useful information about themselves, such as gender or sexual receptiveness; to change others鈥� physiology, for example by stimulating ovulation; and to directly affect others鈥� behaviour. Examples of these behaviour-changing or 鈥渞eleaser鈥� pheromones include sexual attractants and the alarm pheromones many mammals 鈥� among them rats and deer 鈥� use to put others on alert without giving away their location with an alarm call.

For many years, it was assumed that humans did not produce or respond to any of these types of pheromones. In part that was down to a reluctance to admit that humans might respond in such an 鈥渁nimal鈥� way. There is also no clear mechanism for how they might act on the human brain. In animals, pheromones are usually detected by the vomeronasal organ (VNO), a pair of mini-nostrils inside the nose which detect airborne pheromones and relay messages directly to the brain. Although humans have something resembling a VNO, there are no neurons linking it to the brain. And while we have the genes required for a working VNO, they no longer code for functional pheromone receptor proteins (New 杏吧原创, 17 May, p 42). The obvious conclusion was that we had lost this method of communication at some point in our evolution.

That hasn鈥檛 stopped researchers from making claims about human pheromones. In 1971 Martha McClintock, a social psychologist at Harvard University, famously reported that women who live together gradually synchronise their menstrual cycles. McClintock, now at the University of Chicago, suggested that this might be mediated by pheromones (). In 1998, along with her Chicago colleague Kathleen Stern, McClintock found evidence to support this idea, showing that sweat from women in different stages of their menstrual cycle either extended or shortened the cycles of other women (). Despite this evidence, McClintock鈥檚 conclusions remain contentious because nobody has yet isolated the actual chemicals that cause the effect.

Sexy scents

More controversial still are claims that pheromones are involved in human sexual attraction. This is partly due to some high-profile yet scientifically questionable research done by David Berliner and Luis Monti-Bloch of the University of Utah in the mid-1990s. They claimed that when they exposed people to reproductive hormones from the opposite sex, they could see an electrical response from the part of the nose where the vomeronasal organ would be.

In women, cells in this region responded most strongly to extracts containing androstadienone, a hormone related to testosterone found in male sweat. Men reacted similarly to estratetraenol, which is found in female urine. The same team also reported that releasing these compounds into the air subconsciously altered mood, having a calming effect on the opposite sex. The research was widely greeted with scepticism because of Berliner鈥檚 financial interest in a brand of perfume called Realm, which was laced with the supposed pheromones. The team鈥檚 insistence that the pheromones act through the VNO also raised eyebrows. With no convincing evidence for a functional human VNO, many people dismissed the research outright.

Despite this chequered past, there are still plenty of researchers who insist that human pheromones are alive and well. One is Johan Lundstrom, a neuropsychologist at the Monell Chemical Senses Center in Philadelphia. He has shown that women can consistently sniff out their sisters from friends and strangers even when they are not consciously aware of any difference in odour. A similar effect is our well-known ability to select mates based on genetic signatures evident in their body odour 鈥� a subconscious indicator of the compatibility of their immune system.

To Lundstrom, this is convincing evidence that human pheromones are real 鈥� it鈥檚 just that most researchers have a blind spot when it comes to using the P-word. 鈥淚f you asked scientists whether one human can convey some sort of social message to another in their body odour, 99.9 per cent of them would say yes,鈥� he says. 鈥淚f you ask them if humans have pheromones they鈥檒l say 鈥榯hat hasn鈥檛 been demonstrated鈥�. It鈥檚 a semantic issue.鈥�

That may all be about to change, thanks in part to the recent discovery that some animals detect pheromones using their normal olfactory system, not the VNO. 鈥淭here are several well-established examples of pheromone communication in animals that don鈥檛 require the VNO,鈥� says Meredith, who specialises in the animal VNO. For example, a recent study found that mice detect alarm pheromones via a bundle of nerve cells at the tip of their nose that is wired into the normal olfactory system ().

Brain-imaging studies have also helped revive the idea that humans may respond to sex pheromones. In a series of recent experiments, neuroscientist Ivanka Savic of the Karolinska Institute in Stockholm, Sweden, exposed people to androstadienone and watched what happened in their anterior hypothalamus, a part of the brain thought to be involved in sexual behaviour.

In one study, published in 2005, Savic found that androstadienone activated that brain region in heterosexual women and homosexual men, but not in heterosexual men or homosexual women. She later found the opposite effect with estratetraenol.

Brain-imaging studies are also providing tantalising evidence that humans release and respond to alarm pheromones. These have been less well studied than those thought to be involved in attraction, but a handful of psychological studies have claimed to show that humans can detect the 鈥渟cent of fear鈥�. In 1999, Denise Chen, a psychologist now at Rice University in Houston, Texas, asked a group of volunteers to sniff sweat from people who had watched either funny or scary film clips. More than half of the volunteers successfully identified a sample of fearful sweat despite not being able to consciously smell any difference in the samples.

In a similar study in 2002, Kerstin Ackerl from the University of Vienna in Austria reported that women seemed to be able to detect the scent of fear. The 60 women rated sweat from women who had watched a scary movie as stronger, less pleasant and smelling more 鈥渓ike aggression鈥� than sweat from women who had watched a neutral movie ().

Yet these studies used relatively small sample sizes and failed to control for factors such as how strongly different people reacted to the scary movie. They also tended to use questionnaires that asked leading questions about whether the sweat smelled like someone who was happy, angry or scared.

Now, though, a study by Lilianne Mujica-Parodi at Stony Brook University in New York may have eliminated some of these problems by looking directly at what 鈥渇ear鈥� sweat does to the brain.

In the study, which is yet to be published, the team taped absorbent pads to the armpits of 40 volunteers who were about to do their first ever skydive, to collect their sweat as they hurtled towards the ground. Back in the lab, the team transferred the sweat 鈥� plus samples of normal, fear-free sweat 鈥� into nebulisers and asked a second group of volunteers to breathe samples while lying in an fMRI scanner, without telling them anything about the nature of the experiment. Sure enough, volunteers showed significantly more activity in the brain鈥檚 fear centres, the amygdala and hypothalamus, when breathing fear sweat.

鈥淭hey put pads into skydivers鈥� armpits to collect sweat as they hurtled towards the ground鈥�

It鈥檚 not clear whether the volunteers actually felt scared after breathing it in 鈥� the researchers didn鈥檛 ask them in case this biased the results. But the team say the fact that their fear circuitry lit up in response to the putative pheromone 鈥渋ndicates that there may be a hidden biological component to human social dynamics, in which emotional stress is, quite literally, 鈥榗ontagious'鈥�.

Taken together, Savic and Mujica-Parodi鈥檚 work adds considerable weight to the idea that humans produce and respond to pheromones. Yet many people feel it is too early to declare a truce in the pheromone war.

Lundstrom says there is still a missing piece of the puzzle. 鈥淭o my mind, activation of brain is not enough,鈥� he says. 鈥淚 use brain imaging in my work but I like to see that there is a behavioural response 鈥� and to see it consistently. Not just once, but every time.鈥�

To provide this evidence, any human pheromone will need to be identified, synthesised and 鈥� crucially 鈥� tested to see whether it triggers a reliable behavioural change. Only then will we be able to say definitively that human pheromones exist.

This is a tall order. As Lundstrom points out, human body odour contains more than 2000 different compounds. 鈥淧icking one is like putting a blindfold on someone, spinning them around and asking them to hit the centre of a dartboard,鈥� he says.

If human pheromones are identified, however, a whole new debate would open up about how they could be used. Some uses raise more concerns than others. Spraying on some bottled sex appeal may be sneaky in the battlefield of modern dating, but the prospect of synthesising a substance that can induce fear in others is another matter entirely, not least because Mujica-Parodi鈥檚 research was funded by DARPA, the US military鈥檚 research arm. After seeing the team , one blogger pondered whether the military was planning to use pheromones to send people 鈥渟tampeding like spooked cattle鈥�. DARPA, however, says it is not aware of any military plans for fear pheromones and has no plans to fund further research in this area.

According to Simon Wessely, a psychiatrist at the King鈥檚 Centre for Military Health Research at King鈥檚 College London and a health consultant to the British army, the idea is also scientifically implausible. He points to studies done by Stanley Schachter at the Massachusetts Institute of Technology in the 1960s, in which people were injected with adrenalin to create the physical symptoms of fear. These people only became fearful if the situation became threatening, suggesting that context is crucial. 鈥淵ou can generate the physical symptoms of fear but people don鈥檛 necessarily get scared,鈥� says Wessely.

Similarly, Lundstrom has found that women only show a reliable response to androstadienone if there is a man in the room at the time of exposure. And since none of the subjects felt compelled to jump on the male in question, it seems fair to assume that in real life any pheromone-induced effects are likely to be small and influenced by other factors ().

As for whether it鈥檚 worth investing in pheromone-laced aftershave for that big night out, Lundstrom sounds a note of caution. 鈥淢ost of these companies are selling andosterone 鈥� it鈥檚 a pig pheromone that 60 per cent of people can鈥檛 smell and the rest think smells like urine,鈥� he says. On balance, it鈥檚 probably worth working on some witty conversation instead.