Do human sex pheromones exist?

Crédit photo Sven-Kåre Evenseth (CC BY-NC-ND 2.0)

Among the discourses that claim to be biology about human sexual behavior, the one that gives a prominent role to pheromones in sexual attraction is not the least. Yet there is still no consensus among scientists to consider not only that they would play a role, but even that the human species would produce them.

What is a pheromone?

In 1959, biochemist Peter Karlson and zoologist Martin Lüscher proposed the word “pheromone” to designate a class of biologically active substances that were similar to hormones but could not be included among them. Thus, if hormones such as pheromones are produced by glands, the former transmit a chemical message to cells within the body, while the latter transmit a chemical message to other individuals of the same species. This essential difference between hormones and pheromones is due to their mode of production: by endocrine glands (which secrete substances directly into the bloodstream) for hormones; by exocrine glands (which secrete substances intended to be evacuated from the body to the external environment) for pheromones.

To sum up, hormones have a role in regulating metabolism, while pheromones influence the physiology (in bees, the pheromonal production of the queen inhibits the ovarian activity of workers) or the behaviour, especially sexual behaviour, of other individuals of the same species.

What are the properties of pheromones?

Five criteria are generally used to describe a pheromone: 1) the simplicity of the chemical signal; 2) a behavioural response to the stimulus that does not vary and whose function is evident; 3) an almost exclusive reaction (high selectivity) in the stimulus-response coupling (no or few “parasitic” reactions); 4) a species-specific response; 5) the absence of learning in the coupling between the stimulus and the response.

Pheromones thus trigger stereotyped, predetermined and very “framed” behaviours which are not the result of learning, and which are triggered within the same animal species (and not from one species to another).

How are pheromones detected?

In insects, pheromones are usually detected by antennas. In vertebrates, and more particularly in mammals, they are perceived by an olfactory system called accessory, consisting of an organ called vomeronasal, located at the level of the nasal septum which separates the nostrils (nasal septum). This olfactory system differs from the so-called main olfactory system, which captures odours.

Are pheromones odours?

This distinction between olfactory systems shows that, contrary to the common representation, pheromones are not odours. Yet confusion is regularly made, including by some scientists. Its location on the nose is not for nothing in this. Moreover, the very fact of speaking of the vomeronasal organ as the main element of an olfactory system tends to maintain this confusion. But if olfaction is commonly associated with the sense of smell, it refers to the fact of capturing volatile molecules, which are both the odorous molecules and some pheromonal molecules (other pheromonal compounds are not volatile and are transmitted for example by contact).

What about pheromones and the vomeronasal organ in humans?

The issue of pheromones in humans, particularly sexual pheromones, is controversial for more than one reason: (1) the presence of a functioning vomeronasal organ; (2) nonacquired stereotyped behavioural responses; and (3) the production of pheromones and the substances defined as such.

Scientific studies conducted to locate the vomeronasal organ in human beings generally lead to the conclusion that it is present only as a state of vestige and that it is not functional. In particular, genes coding for pheromonal receptors have been shown to be inactive (in humans in particular, but also in other vertebrates). In addition, the nerve bundles that provide the neuronal link between the vomeronasal organ and the brain are absent. This has led some scientists to hypothesize a perception of pheromones by the main olfactory system, and thus to bring pheromones closer to odours. However, such a hypothesis requires several deviations concerning the definition of pheromones.

Few studies on pheromones actually focus on human beings, and many hypotheses about human behaviour are extrapolated from studies on other animal species (insects, rabbits, mice…). But the few who wanted to test hypotheses about human beings could not establish that putative pheromones provoked stereotyped and predetermined behaviours. At best, mood variations could be observed within small groups (10 to 30 people), not uniform and attributed by hypothesis to the perception of a steroid hormone (whereas pheromones are supposed to designate molecules that cannot be reduced to hormones), without being able to ensure that these variations were not due to other factors.

In the end, it is the very production of pheromones by human beings that is not proven. If the presence of remains of a vomeronasal organ may lead to the hypothesis that the human species may have produced them at a certain stage of its evolution, no chemical substance meeting the definition of a pheromone and grouping its properties has been identified in today’s human beings. The shift towards steroid hormones and odours – otherwise captured by other species – to maintain the hypothesis of pheromonal action in humans only highlights that the existence of a functional human pheromonal complex, consistent with what the term “pheromone” is supposed to mean, could not be attested.

However, the role of odours on the psychological state, and in particular on sexual attraction, seems to be proven. This is the case of odours associated with steroid hormones, but also artificial perfumes, which can cause desire or repulsion. But on the one hand feelings and reactions vary from one person to another, and on the other hand smells are only one factor among many that come into play in sexual attractions or repulsions. As biologist Patricia Nagnan-Le Meillour reminds us, in human beings “the choice of partner is strongly conditioned by belonging to a social class or by other sociological pressures, leaving little to attraction via chemical signals, even if they cannot be totally denied”. To this are added psychological factors that lead us to partners with whom we will repeat psychological issues and acquired relational modalities. The human sexual attraction determined by pheromones thus does seem, finally, to be only a myth.

For further information: “The controversy about the role of pheromones in sexual attraction between humans”