The heady scents of attraction - the powerful allure of pheromones

A confusing aroma

Pheromones can be set apart from simple odours in that they serve as airborne signals between individuals of the same species. Although they may have no discernible odour, pheromones do stimulate marked physiological or behavioural responses within another individual. Charles Darwin was the first to speculate about the existence of pheromones when he noted that horses and bulls raise their nostrils and breathe excitedly in the presence of a receptive female, although he observed that dogs and cats also used scents to mark their territory and announce their presence. However, the first pheromone to be isolated in 1956 was a potent sexual attractant released by female silkworm moths. A tiny quantity of this pheromone was sufficient to cause male moths many miles away to beat their wings frenetically. However, Paul Broca, a leading 19th Century neuroscientist, argued that the region of the brain which is associated with smell, the olfactory cortex, is far smaller in humans than in other animals, suggesting a greatly reduced importance of our sense of smell. This notion appealed to a Victorian world which placed man above all other animals, as this made the olfactory region of the brain, located just behind the nose, merely a primitive stage in our evolution towards becoming sentient, visual beings. Further doubt was cast upon the existence of human pheromones when scientists claimed that they were unable to find the tiny sensory pits inside the nose which are responsible for the detection of pheromones, a region known as the ‘vomeronasal organ’ or VNO. These ‘observations’ effectively buried any scent of pheromones as a potential mode of human communication.

However a doubt still lingered in the air. Many social scientists observed that smelling the faces or hands of others appears to be a universal human greeting, although it has evolved into different forms within various cultures, such as the rubbing of noses by Eskimos or the kissing of both cheeks in France. This is perhaps no pointless custom, as the hands and face provide the highest accessible concentrations of scent glands on the human body.

Whilst the power of certain scents and perfumes to influence mood and emotion has never been in doubt, it is of course exceptionally difficult to prove the existence of odourless pheromones that affect emotion and behaviour. Knowing that we display strong emotional reactions to the smells of others, and given that we as humans produce many scented substances within our sweat, skin oils and sexual secretions, it could be suggested that the human VNO might not need to be very large to register the scent of others, especially as we possess higher densities of scent glands within our skin than most other mammals. The mystery was finally solved in the mid 1980s when scientists discovered two tiny pits located an inch inside the human nose, the human VNO.

So here lies the problem. How do we prove that odourless, airborne substances are released from the human body, detected and subsequently act to produce changes in our mood or emotions which are distinct from the effects of a scented perfume? The only way to distinguish the effects of odours from those of pheromones was to prove that the human VNO can detect odourless compounds released by glands on the human skin, and to demonstrate that such pheromones trigger a response within an appropriate region of the brain that is known to influence mood, perception or behaviour. No small order, but here the beautiful logic of science is elegantly demonstrated. Scientists first showed that the tiny pits of the VNO are lined with receptor cells that become electrically hyperactive when presented with a candidate pheromone. Although these pheromones did not trigger a sensation of smell, they did confer a sense of warmth, comfort and well-being in individuals who were tested. It was also found that the human olfactory bulb, the region of the brain which detects and processes pheromones, was not so much missing, as buried beneath our greatly expanded frontal lobes. However science is fuelled by controversy, and there are those who believe that our conventional site of smell, detected by the olfactory lining of the nose, detects those pheromones which cause sexual attraction, whilst the newly discovered VNO detects those pheromones which have repellent actions. OK, so at least we have now established that there is a human pheromone detection and processing system, but what pheromones do we produce, which glands release them, and what do they signal?

We possess three types of gland within our skin; sebaceous, sweat and apocrine glands. Our sebaceous glands release a mixture of oils, antibiotics, and possibly pheromones. Sebaceous glands are particularly concentrated around the face and forehead, although they are also found surrounding all of the body’s openings, including the eyelids, ears, nose, lips, and areolae (nipples). Sweat glands release salt, urea and water and are not thought to produce pheromones. The most likely source of human pheromones is the apocrine gland. Apocrine glands are present in dense concentrations upon the hands, cheeks, scalp, nipples and hairy regions, and become active only after sexual puberty. This is not all good news. The tufts of hair in the genital regions and other regions of the body provide a large surface area from which apocrine secretions may become airbourne. Unfortunately for womankind the apocrine glands of men are larger than those of women, and these glands tend to secrete more during periods of nervousness or excitement. Hoardes of bacteria lie in wait to convert these apocrine secretions into the undesirable odours that fuel a $billion deodorant industry. If pheromones are an important mode of human communication this appears to be the social price.

When we are emotionally or sexually excited we release pheromones through our apocrine glands located in the arm pits and genitalia. This may explain the erotic gesture of a woman who sighs and raises her arms to her head, a movement long considered to be sexually alluring. Odours are believed to play a key role in sexual excitation and attraction, since sniffing diverts blood and air to the nose and creates a turbulent air flow, maximising the availability of a scent. Women have a superior sense of smell to men, and this sensory ability is known to peak around the time of ovulation. The deliberate coincidence of ovulation and a heightened sense of smell aid the detection of airborne pheromones, which in turn serve as key indicators of male attractiveness and fertility.

The role of scent in human behaviour

So what aspects of our mood and behaviour may be influenced by pheromones? Pheromones may allow mothers to recognise their own infants, ensuring that only their offspring receive maternal milk. Both male and female pheromones have been shown to promote and synchronise a woman’s fertility cycles. It is likely that certain pheromones may actually serve to repel sexual rivals, in addition to those pheromones which are believed to be sexually attractive. Women who have had a hysterectomy or ovariectomy are known to become less attractive to males - a condition which may be experimentally reversed by the application of pheromones. So what do the various actions of phermones all have in common? They all play a role in sexual competition, selection and reproduction. To put things simply, pheromones are airborne sex hormones that are common to all species, from moths to humans. Individuals with a high degree of facial symmetry release more pheromones, in effect serving as subconscious indicators of a person's readiness and capacity to reproduce. Intriguingly, a woman’s level of pheromone release appears to correspond closely with her reproductive success rate.

Throughout history certain scents have long been held to be attractants or aphrodisiacs. In ancient Rome perfume jars were kept in bed chambers to increase sexual desire, Egyptians bathed with essential oils in preparation for sexual intercourse, and even the ancient Sumerians record how they seduced women with scents. Indeed almost all cultures have used perfumes within their marriage ceremonies and reproductive rites. The perfume industry is today a multi-billion dollar business which spends fortunes upon research. Naturally they are keen to bottle the elixir of instant love. Even Sigmund Freud recognised the importance of body odours, suggesting that human scents were strong inducers of sexual desire. Being from the Victorian Age, he also believed that the repression of smell sensations was essential for civilization and for civilised behaviour.

Human experimentation is as informative as it is controversial, and certainly shows how important social ranking is to our behaviour and chances of attracting and securing a sexual partner. Perhaps the most famous pheromone experiment was a chance discovery, as many of the most important discoveries are. It was observed that women who live within all-female dormitory halls or convents quickly synchronize their ovulation cycles with that of the dominant female within two months of their arrival. This synchrony is just the tip of the iceberg. Male college students were asked to evaluate pictures of women whilst wearing masks laced either with the theoretical female pheromone androsterone, or without it. Pictures received significantly more favourable assessments when the scented masks were worn, and this might explain why some men have been arrested for stealing women’s underwear. In another classic experiment, a suspected male pheromone was placed under certain desks in a classroom, and the seating preferences of male and female students were recorded on film. Female students subconsciously appeared to prefer to sit at or near desks where a theoretical male pheromone was placed.

Pheromones are believed to signal not only sexual attraction, but also to mark sexual territory. In a men's college dormitory a male pheromone was placed underneath specific toilet stalls which were then largely avoided by other men. Indeed medical science points towards an association between the sense of smell and sexual health, as individuals who have a diminished sense of smell are more likely to develop sexual dysfunctions. At the peak of ovulation women tend to produce more estrogens, and this seems to correlate strongly with their attractiveness to other males, an attraction which is communicated by pheromones.

Sexual competition and prowess - the power of male pheromones

Dr. Winifred Cutler's classical studies of the 1970s suggested that women who have regular sex with men demonstrate more regular menstrual cycles than those who engage in occasional or casual sex. Women who had more regular heterosexual intercourse also experienced a delay in the decline of their estrogen levels and were correspondingly more fertile. So men might be more useful to women than many feminists had previously believed. One psychological study determined the effects of secretions from the male armpit upon female estimations of male sexual attractiveness. The findings depended upon the stage of the woman’s menstrual cycle, although women exposed to male secretions generally found male images significantly more attractive. So what might be the identity of this potent, yet elusive essence of man, this eau de male locker room?

Apocrine glands are known to release a bouquet of pungent steroids that elicit sexual behaviours within other mammals. At puberty, human apocrine glands begin to produce steroidal secretions which contain androstenone and androstenol. These substances are derived from ‘androstenes’ via conversion of the male steroid hormone testosterone. Certain bacteria then act upon released androstenone and androstenol to produce further compounds. Androstenes have been confirmed to play a role as male sexual pheromones in pigs, and several researchers have suggested that androstenes and their derivatives function as human male pheromones in both attraction and male dominance ranking.

So which of these steroids are the real male pheromones, and can we really bottle their essence? One of these, androsterone is released in larger amounts by men than women, and moreover, the majority of men are unable to detect androsterone until puberty – the point at which its production commences. In further support of its role as a potential pheromone, the reactions of women to androsterone appear to change during ovulation. So far, so good, for our candidate human pheromone. However, androsterone is particularly foul smelling, and instead of being attracted by androsterone during ovulation, women were at best indifferent to it. So there we have it, androsterone is not sexy, at least to human females.

A second promising candidate pheromone is androstenol, is a steroid found in fresh sweat which has an aroma said to be reminiscent of ‘sandlewood'. Several scientific studies have indicated that when exposed to androstenol female ratings of male attractiveness are considerably higher. It is shown that both heterosexual women and homosexual men prefer seats sprayed with androstenone, a component of sweat degradation. In contrast, androstenone enhanced both the positive or negative feelings of heterosexual males towards other men. So there are at least three pheromones involved in sexual attraction, androstenol and androstenone which promote female attraction to men, especially during ovulation, and androsterone, which induces negative responses in females.

Does this signal a prospective gold rush as scent manufacturers rush to produce a range of pheromone-based scents to attract women? It may not be quite so straight-forward. Whilst many quote a famous experiment in which women apparently preferred a pheromone-sprayed chair in a waiting room, the problem is that male pheromones only have an effective range of one or two feet. A further point for reflection is that a pheromone only serves to influence the perception of an object, so whilst pheromone-based scents may have an arousing effect upon women, the judgement of a woman must be directed towards an individual or they will be unaware of the source of their arousal. A man wearing a pheromone-laced scent at a crowded party will still have to compete with the other men present for the attention of a woman. Only within an intimate encounter may the arousing effect of the scent actually benefit its owner - and to achieve such an encounter, the man must presumably be capable of attracting the woman by some other means, be it physical, social or material. No amount of pheromone is going to make an unattractive man stand out from the crowd.

The fertility hormone - finding the female pheromone

The first evidence that female pheromones may serve as sexual lures came from studies in monkeys in the 1970s. It was observed that female monkeys who had lost the function of their ovaries were unable to attract the attentions of males. Their female power was however duly restored upon replacement of a feminine musk containing pheromones. Around the same time it was noted that nuns in convents and young women in college dormitories all began to menstruate at the same time. As Dr. McClintock put it “women who live together, cycle together”. In their search for the elusive female pheromone, scientists have isolated and detected over a hundred organic compounds from the underarm sweat of fertile women, and they have used this essence to reproduce the classical menstrual cycling experiments. When this extract was added to a perfume formula, a significantly larger proportion of those women and men who wore the pheromone-laced scent engaged in kissing, petting, intimacy and sexual intercourse than those subjects who were sprayed with a placebo.

Female pheromones are known to be produced by the armpits. These pheromones increase activity within a specialised region of the brain known as the hypothalamus, which is followed by sensations which include an elevated heart rate, perspiration, and a general feeling of sexual arousal. The hypothalamus serves to direct hormonal responses to the many changes that occur within the body’s external and internal environment, including pheromones. The identity of the female synchronising pheromone derived from progesterone remains elusive, although its presence within the sweat of women suggests that it does not serve as a sexual attractant for other women, although the unnaturally high concentrations used in experiments may have thrown researchers off the scent. However, an estrogen-based pheromone may provide a better candidate as a sexual attractant for males, as it was this compound, rather than the progesterone-like pheromone which excited the hypothalamus in men, an area which also governs sexual behaviour.

The power of scent over sentiment - the subconscious influence of pheromones

Many scents, whether they come from humans, animals or foods, evoke powerful feelings of desire, happiness or revulsion. However, our sense of smell is linked to far more than our mood - it is also strongly associated with the formation of memories. Indeed our sense of smell colours memory, and a single aroma can evoke past memories of times and places. Aromatherapy is based upon the idea that mood and well-being are affected by a combination of scents and pheromones, or aromas. It seems that pheromones add emotional tone to our memories, as well as affecting our mood and judgement. Scientists have also been establishing that scent plays an important role in whether we perceive women to be attractive. At certain points within their monthly cycle women produce more or less estrogen, and this makes their scent accordingly more or less appealing to males.

Our sensitivity to pheromones seems is an ancient and physiological part of what makes us human, encoded within our genes and processed within the subconcious region of our brains. Sexual differences in our responsiveness to pheromones also appear to be hard-wired, as the brains of men and women appear to react differently to two candidate pheromones. An estrogen-like compound triggers an increased blood flow to the hypothalamus in the brains of men, but not those of women. Increased regional blood flow in the brain is taken to represent increased activity in that part of the brain, and the hypothalamus governs sexual behavior by stimulating the release of ‘master’ sex hormones from the pituitary gland which lies just below it. In contrast, an estrogen-like pheromone had no effect upon the olfactory region of the brain governing smell. In contrast, a prospective pheromone present within male sweat had the exact opposite set of actions, activating the hypothalamus in women and the olfactory regions within other (heterosexual) men.

However, the brains of homosexual men responded to these same two pheromones in a manner which was identical to that seen in women. The issue of gender and sexual orientation does not rest there, as this specific region of the hypothalamus is twice as large in heterosexual men as it is in women or homosexual men. A similar study in lesbian and heterosexual women showed a similar pattern in response to pheromones related to progesterone and estrogen. The brains of heterosexual women showed activity within the anterior hypothalamus in response the progesterone-related pheromone, whilst the estrogen-related pheromone produced an increased activity only within the olfactory region. However in lesbians, only the olfactory region responded to either compound. The pattern of activity observed within the brains of lesbians was similar to the patterns seen in heterosexual men. So whether an airborne steroid acts as a pheromone or as an odour depends upon your sex, or rather your sexual orientation.

Opposites attract

One popular theory is that a male’s body odour smells attractive to a woman if he possesses a complementary set of genes, especially those which promote immunity to disease. During ovulation women find the body odours of men with perfectly symmetrical bodies more pleasant, as they are indicative of good breeding and inheritance. In mice the female selects the male whose immunity gene profile is least similar to her own, a genetic legacy which is apparently encoded in detail by pheromones. Favourable smells signify potential sexual partners who are more distantly related, and therefore represent better genetic matches. In contrast, unpleasant smells usually indicate a closer relation. So what bearing does all this mouse business have upon us as humans, and upon our own sexual preferences? Most women find the smell of their own close male relatives displeasing, and this mouse research offers a tempting explanation. Indeed human experiments suggest that women find men with dissimilar immune profiles more enticing than those who share a more similar genetic make up.

One point of major social concern is that oral contraceptives are causing a reversal in our sexual preferences, with women suddenly preferring men with a closer genetic heritage. In fact the contraceptive pill may even be responsible for increasing the divorce rate, as many wives who suddenly stop taking their contraceptive pills begin to find their husband's body odour distinctly distasteful. This may be the beginning of the end for the relationship, as it subconsciously reveals a genetically incompatible mate. The plot thickens. Couples who experience difficulty conceiving a child appear to share a significantly higher percentage of certain genes which encode for proteins of the immune system than those partners who conceive more readily. Some scientists believe that infertility problems in couples may be due to subconscious reproductive choices made by women to avoid having children with a more susceptible immune system.

It appears that selecting suitable mates through pheromones may not be a primeval mechanism from which human evolution has advanced. In fact, rather than using this ancient, hidden reproductive code, Westerners may have lost many of its advantages due to the evolution of a culture in which we are obsessed with regular washing, perfumes, contraceptives and deodorants. This may even help to explain our falling fertility rates, rising divorce rates and the growth of IVF medicine. It certainly explains why our belief in human pheromones has declined in recent centuries. On the other side of the equation, signals from male pheromones which trigger feelings of fear or aggression in other men have also been diminished, reducing the likelihood of social conflict.

More surprisingly body odour may even give away inherited disease. Those with diabetes, viral infection or mental illness are apparently revealed by their body odour. This will give the perfume manufacturers an additional headache. So before you rush off to the cosmetics counter and order an expensive bottle of cologne or perfume laced with pheromones, you would do well to remember than the signals that you might be giving off have yet to be fully decoded...