Can men smell a pretty woman from afar, and do women seeking a partner desire a man who smells like their father? Jerome Burne sniffs around controversial findings in phermone research.
Smell has always been the poor relation of the senses. Philosophers right back to Aristotle have dismissed it as the basest, most primitive one. Kant ranked it way below the noble sense of sight on the grounds that it was the most subjective. It was Freud who, as it were, smelt a rat and argued that the philosophical rejection of smell concealed a fear of its power. When humans began walking on two legs, he suggested, we left behind the world of smell, of scent trails and females on heat. Instead we came to rely on vision, which allowed us to gain an abstract knowledge of the world. In return for repressing our noses, we gained civilisation.
Recent research, however, shows that smell is still a big player with great manipulative powers, capable of making us feel sexy, relaxed or alarmed. But how exactly it works is the topic of furious and fascinating debate. Only last month, Norma McCoy of San Francisco State University announced the results of a double-blind study that found that adding a patented mixture of pheromones to subjects' regular perfume increased by 73 per cent the number of "sociosexual behaviours" - from kissing to sex - they engaged in during a week.
This is controversial because some researchers still doubt whether humans really respond to pheromones - the odourless chemical messengers that control much of animal sexual behaviour. The merest whiff of boar pheromone, for instance, sends an ovulating sow into a mating frenzy. The organ most mammals use to register pheromones is the VNO (veromonal nasal organ) - two tiny pits inside the nostrils separate from the olfactory epethelium, which detects smells. But do humans have a working VNO?
No, says Ursula Goodenough of Washington University in St Louis, Missouri. "Humans have only a vestigal VNO, and all but two or three of our pheromone receptor genes are pseudogenes," she says. Goodenough also paints an intriguing evolutionary scenario that both supports and demolishes Freud's theorising. We lack a copy of a gene called Trp2, she explains, which is needed for the pheromone system in rodents. Primates used to have one until the split between Old and New World monkeys millions of years ago, when our primate ancestors lost them. About the same time, however, the green receptor appeared in the visual system, improving vision. So maybe smell was sacrificed for vision, but rather earlier than Freud thought. Other pheromone researchers, such as Harvard biologist Catherine Dulac, are equally sceptical about the existence of a human pheromone system.
Confronting the sceptics are a number of researchers who are unworried about the anatomical details and convinced that pheromones play a vital role in human sexual life. They point to studies such as those conducted by Suma Jacob of the University of Chicago, who has found that the odourless steroids androstadienone and estratetraenol, produced by men and women respectively, can directly affect moods. Curiously though, although the masculine one cheered up women, the female one irritated men.
In July last year, another Chicago researcher, Martha McClintock, made brain scans of volunteers who had been given these steroids. The results showed that areas that processed smell were active but so were those concerned with attention, emotion and vision. "This supports the notion that androstadienone modulates ongoing behaviour," McClintock says.
What is much less controversial is that the natural smell that each of us has can have quite specific effects. Women seem to come out of this research rather better than men. The scent of granny - rated from pads taken from the armpits of post-menopausal women - was recently found generally to make people happy, while "mummy odour", taken from mothers of newborn babies and said to resemble sandalwood, can act as a cure for mild depression. Attractive women do well, too - even when men cannot see them, they rate their smell as more desirable than their plainer sisters. But, as if they did not have enough problems, the smell of teenage males makes people feel angry.
Although women's sense of smell is typically far more acute than men's, they are not as good at picking out good-looking partners by smell alone. They are fine when they are ovulating, when their sense of smell is keenest, but around menstruation their powers tail off. But smell does not just provide information about superficial attractiveness. Several years ago, Claus Wedekind of the University of Bern uncovered a direct link between a man's smell - on a T-shirt he had slept in for two consecutive nights - and the working of his immune system.
In the study, women preferred the smell of shirts belonging to men whose immune system had the smallest overlap with their own. The sexiness of the shirts correlated with the degree of difference between the woman's and the man's MHC (major histocompatibility complex) genes, code for the disease detectors in the immune system. In other words, women were most attracted to men whose MHC genes were least like their own. The implication is that, should the pair have a child, it would have the widest possible range of immune responses.
Last year, Wedekind reported that the sort of perfume people preferred related to their MHC, suggesting that perfumes, far from suppressing our natural smell, enhance it.
That all seemed remarkable and eminently sensible, but that neat theory was upset in January of this year when the Chicago group reported that the smell that women were really drawn to was that of Daddy, or rather to men who had a MHC similar to his. So what do women want? Same or different? The confusion may arise from the way the question was phrased. The women were asked which they would choose "if they had to smell it for the rest of their lives". So it was not clear if they just liked Daddy's smell or whether they found it sexy.
A lively debate has also been raging around McCoy's research on women and pheromones. She claimed that pheremonally enhanced perfume increased women's sociosexual behaviours because the pheromones had influenced men, making them more responsive. But David Leake of the University of Hawaii has pointed out that it is generally female choice that controls sexual events. Men rarely need encouragement, but women tend to be more sexually responsive when they are ovulating. So could the pheromones promote this feeling? There is still plenty of mystery in the invisible and often unconscious world of sexual scents.