Smelling each other

Parem molta atenció a què ens agrada o ens desagrada d’altres persones; especialment si les considerem possibles candidats a ser la nostra parella. Tot i que no és tan freqüent que fem referència a la seua olor, potser es tracta d’un dels elements més importants a l’hora d’establir si dos persones seran o no compatibles. Des del punt de vista immunitari, si més no, hi ha molts elements per a creure-ho. El Complex Major d’Histocompatibilitat, la font principal de feromona humana, pot condicionar molt decisivament el grau en què ens sentim atrets per una altra persona. És fàcil entendre com la indústria del perfum pot estar interessada en la investigació sobre aquest aspecte de la sexualitat humana. Podrà traure’n profit?

We tend to assume that we like or dislike people because of their behavior. If asked for it, we would very probably be able to mention a lot of things we like in our friends or loved ones. Being respectful, warm, intelligent, funny or faithful might be among the features we would come up with to refer to someone we love. And of course we would probably also recall their physical look (tall, handsome, attractive, beautiful eyes…) and even the pitch of their voice or the way they use language.

 
But what about their smell? The perfume they wear or the subtle (or not so subtle) personal aroma which -with not so laudatory intention- we might refer to as body odor (BO) is a sensory characteristic of every one of us. In some cases its impact is undeniable. Some smells can attract us in a noticeable manner. Some other smells we wouldn’t hesitate to describe as unbearable and to run away from them as politely as we can manage. But would we readily admit that we like someone primarily because of their BO?
There are probably very few people willing to assume such a statement. Nevertheless, scientific evidence points to a decisive role of our noses in such an important matter as choosing mates.
 
The existence of human pheromones (from phero, Greek for “to bear” and hormone, Greek for “impetus”) was denied for a long time, mostly because unlike other animals, our brains lack a vomeronasal organ (VNO). In animals, this accessory olfactory bulb detects conspecific individuals’ pheromones and is largely responsible for sexual behavior. It seems, though, that even without a VNO, we humans manage to respond to our conspecifics’ pheromones through our main olfactory bulb which connects our nose to the core of our brain.
 
The impact of pheromones on human behavior has been demonstrated in many ingenious experiments. One showed that women systematically preferred seats that had been impregnated with masculine fluids known to contain male pheromones. The interesting point is that none of the women reported having chosen a particular seat because of its pleasant odor. 
 
Although men generally have much less sensitivity to odors than women, it seems they are not completely without it. In a study in which several men smelt T-shirts used by women in different phases of their period, they tended to systematically prefer those that had been worn during fertile phases (Singh & Bronstad 2001). 

According to these and many other studies
, our pheromones would act at an unconscious level. It’s not like the men of the previously mentioned study would say something like “What a wonderful fertility smell!” It would be more like odors somehow affecting or even directing our behavior in a subtle way, while we remain oblivious of their presence. 
 
No doubt these results would raise the interest of the perfume industry. After all, there is not a single perfume ad escaping the concept of seduction. Perfumes are not sold because they smell fine but because they -allegedly- make us more attractive and enhance our opportunities to successfully mate.
 
But there is a really interesting point about pheromones which make them quite refractory to the advertisers’ interests. There is not a single chemical compound bearing the ability to be seductive for all of us (not even for most of us). According to our current knowledge, pheromones give us signals (or at least cues) about genetic compatibility.
 
The Major Histocompatibility Complex (MHC), an array of genes involved in the immune response and unique for every individual, is the major source of human pheromones and might be responsible for the failure of the advertising industry to make (legitimate) profits out of selling pheromone perfumes.
For one part, it has to be said that the effects of odor on human behavior are subtle and it would be utterly unrealistic to expect a dramatic effect such as having highly attractive people suddenly rushing into your arms when you wear a particular perfume brand. 
 
But the most important point is that no single odor can possibly be attractive for everyone. This is precisely the great evolutionary value of the MHC, which will produce specific pheromones that will result attractive for people having MHCs which are very different from yours.
 
That way (but just if things go really well after your first date) your eventual offspring will be more ‘protected’ (immune to a wider range of pathogens) and thus, more able to survive and carry your genes down to the next generations. 
That’s why the perfume industry will not be able to market a single product that could legitimately claim to boost your personal attractiveness. If it’s made out of pheromones, people smelling it will feel attracted or repulsed by it as a function of their own unique MHC.
 
Considering that women are much more sensitive to odors than men, we could safely say that our own pheromone identity has not evolved to turn every man into a sort of highly successful playboy but rather to help us, both men and women, to find a really compatible partner, at least from the point of view of immunity. Every time you meet someone eligible as a future partner, it’s up to you to decide if you’re willing –or not- to follow your nose.

(Thanks to Lluís Bosch for his collaboration  

Quant a ferransuay

Professor of  Physiological Psychology and Psychoendocrinology (Departament de Psicobiologia. Universitat de València). Member of the Human Behavior & Evolution Society
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