(a) Other mammals use pheromones, so humans may do so

Many mammals have pheromones that fit the classical definition [5,10,14]. These include the rabbit mammary pheromone 2-methylbut-2-enal (2MB2) [15] and 4-ethyloctanal in male goats (figure 1) [16], as well as protein pheromones such as ESP1 and darcin in the house mouse [12,17].

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Figure 1.

How pheromone molecules can be identified against a background of many hundreds of molecules in a scent sample from a mammal. Scents from the heads of male goats Capra hircus switch seasonally anoestrous females into the oestrous state. The gas chromatograph trace (top) shows the large number of peaks (each corresponding to a molecule) from a sample of volatiles from the heads of male goats [16]. The highlighted section, enlarged (below), was the region that contained the activity, detected in bioassays of female response. As the pheromone secretion was androgen-dependent, Murata et al. [16] looked for molecules missing from the secretions of castrated males (lower, grey chromatogram) and created a synthetic cocktail of 18 molecules (underlined) in the concentrations found in the original headspace analysis of intact males. Most, but not all, of the bioassay activity could be reproduced by one molecule, 4-ethyloctanal, so the pheromone may include other molecules from the 18. Note that the candidate molecules were not the highest peaks. Reproduced from Murata et al. [16] in Current Biology with permission from Elsevier. Goat image, non-copyright.

Darwin [18] noted that adult males of mammals such as goats and elephants have characteristic strong odours during the breeding season. He reasoned that the evolution of specialized odour glands in male mammals is ‘intelligible through sexual selection, if the most odouriferous males are the most successful in winning the females, and in leaving offspring to inherit their gradually perfected glands and odours' [18, vol. 2, p. 281].

If we were any other kind of mammal, the changes in our smell-producing secretions as we develop through puberty to sexual maturity would suggest that these could have a pheromonal role [5]. The changes are due in large part to the development of sebaceous and apocrine skin glands at puberty (see §6). Other primates show similar gland development with sexual maturity and many species from lemurs to monkeys use chemical communication extensively [19,20]. Although we and our nearest relatives, the great apes (the bonobos, chimpanzees, gorillas and orangutans) do not appear to use odour communication as much as other primates, odours are still important to us and we have secretory glands that could be used to produce pheromones [5,20,21].

(b) Humans have a good sense of smell

The possibility of human pheromones has been downplayed in part because in the past it has been assumed erroneously that we have a poor sense of smell [5]. On the contrary, we are, if anything, excellent smellers able to make subtle smell distinctions, though this in itself is not evidence that we have pheromones [19,21–23]. We have a ‘main olfactory system’ but we do not have a functional VNO or ‘second nose’ [24]: however, this is no barrier to our potential use of pheromones as many mammals, including rabbits and sheep, detect pheromones with the main olfactory system [14,25].

(a) Androstenone and androstenol

The finding that molecules with pheromonal effects in pigs were also detectable in human armpits was enough to lead some researchers (e.g. [39,40]) to consider androstenone and/or androstenol as human pheromones [35,37,38,41]. While pheromones do not have to be unique to a species, it does not follow that because one species uses a molecule, other unrelated species are necessarily likely to use it as a pheromone [5]. What likely made androstenone popular with experimenters was the commercial availability of the molecule in aerosol cans as Boarmate™, for use in pig husbandry. Experiments included, for example, spraying the underside of some chairs in a waiting room and seeing which chairs were chosen by women and men. Doty [35, p. 141 ff] critiques many of the experiments in detail, pointing out poor experimental design, statistical errors and small sample sizes. The more fundamental criticism is simply that there was no evidence to justify using these molecules with humans in the first place, rather than any of the hundreds of other molecules found in human armpits (e.g. [42]).

(b) Androstadienone and estratetraenol

These molecules were presented as ‘putative human pheromones' by Monti-Bloch & Grosser [43] at a 1991 conference sponsored by the EROX Corporation, which had commercial interests in human pheromones. The two molecules were among five, described only by code numbers, which had been tested by Monti-Bloch & Grosser [43]. The authors gave no details at all of how these molecules were extracted, identified, bioassayed and demonstrated to be the ‘putative pheromones' of the paper's title, just a footnote: ‘These putative pheromones were supplied by EROX Corporation’ [43]. A later patent, assigned to the EROX Corporation, on ‘Fragrance compositions containing human pheromones' [44], gave no details about how the molecules were arrived at but revealed that the two code numbered molecules claimed to have the greatest and most sex-specific effects on the opposite sex in Monti-Bloch & Grosser [43] were androstadienone and estratetraenol, apparently from men and women, respectively.

These two molecules, androstadienone and estratetraenol, might have rested in obscurity had it not been for a paper by Jacob & McClintock [45] on the effect of these molecules on the mood of men and women. Though the researchers were not associated with the EROX Corporation, the molecules were explicitly chosen because they had been proposed by Monti-Bloch & Grosser [43], Berliner [44] and Monti-Bloch et al. [46].

Jacob & McClintock [45] has been cited more than 150 times (Google Scholar, 23 October 2014) and the molecules and the concentrations used by them have become traditional across much of the field to this day. However, while Jacob & McClintock [45] were fairly cautious, commenting that ‘ … it is premature to call these steroids human pheromones' [45, p. 76], this advice has been forgotten by most later authors.

(c) Positive results from un-evidenced molecules are likely due to false positives, positive publication bias and other problems

The fundamental problem with the experiments using androstene molecules such as androstenone, androstenol, androstadienone or estratetraenol is that, however well designed and carried out the experiments are, there is no scientific basis for treating these molecules as human pheromones. This has been pointed out often from the 1980s to the present (e.g. [35,37,38,41,47]) but the attraction of studies on androstadienone and/or estratetraenol seems unstoppable.

More than 40 papers claiming psychological and/or physiological effects of androstadienone and/or estratetraenol have been published since 2000 (many studies are tabulated in [34]). These range from studies of physiological effects such as Bensafi et al. [48] to the assessment of the gender of computer-generated walking figures [49].

Detailed critiques of some individual studies are presented in Doty [36], Wysocki & Preti [37], and at book length in Doty [35]. More generally, however, because there is no evidence that these molecules have any pheromone activity we should be sceptical about positive results as we have no justified expectation of an effect. Experiments in this and related fields tend to be statistically underpowered, not least because of small sample sizes and overestimates of effect size in the absence of a priori evidence of effects [50–52]. Both the direction and magnitude of the effects in ‘significant’ findings in underpowered studies are likely to be wrong [50,52–55]. Such problems can be compounded by experimenter phenomena such as stopping experiments when significance is reached and flexible statistical analysis [56], combined with positive publication bias (the ‘desk-drawer effect’), when positive results are more likely to be published [51,57]. As in other areas of biology, full replication is rare. In summary, apparently positive results do not, in isolation, demonstrate that these molecules are pheromones.

Even if some of these reported effects should turn out not to be false positives, they do not establish that the androstenes are pheromones: all sorts of non-pheromone odours such as plant odours are reported to have measurable effects on physiology and mood [35, p. 164 ff]. For example, inhaling volatile plant essential oils such as pepper oil, fennel oil or rose oil affects sympathetic activity in adults, shown by changes in blood pressure and plasma catecholamine levels including adrenaline concentrations [58]. In a study of the effects on mood, lemon oil odours reliably enhanced positive mood compared with lavender oil and a water control [59]. However, even these kinds of results can depend on what human subjects are primed to expect (e.g. [60]).

(a) Identifying appropriate bioassays will be a major challenge

A fundamental problem for studying humans is that, with few exceptions (e.g. §7), we have not identified biological phenomena, involving olfaction and potentially mediated by pheromones, that are sufficiently well defined to allow for an unambiguous bioassay [5]. Among the other challenges is cultural conditioning of responses to odours which may render them unimportant or repellent. We know from other mammals, such as the hamster, that responses depend on a complex interaction of previous experience and the context of the message; we should expect human responses to be no less complex [5]. There are many unknowns. For example, if we do have sex pheromones they might be involved in intimate foreplay, not distance attraction. It may be that the first human pheromones to be identified will concern babies not sex (§7).

When we do have appropriate bioassays and have identified well-based potential molecules to test, the expertise and techniques for stimulus delivery and experiments already developed by the many teams currently investigating olfaction in humans and other primates will be invaluable (e.g. [19,74–81]). They have developed careful double-blind designs, controls, sophisticated delivery of molecules, and many refined approaches to measure responses.

(b) The search for molecules needs to cover the whole body

While there are exceptions (e.g. [65,66]), traditionally, human pheromone researchers have tended to look at armpits. There are some good reasons, as axillary (armpit) scent glands are unique to humans and the great apes [20]. Armpits may also simply be among the least embarrassing places to sample.

However, there are many reasons for looking beyond our armpits. First, is that the odours thought by (mostly) Western scientists to be characteristic of armpits are largely absent in most people in northeast Asia, who comprise about 20% of the World's population. About 95% of people in China are homozygous for a variant of gene ABCC11 which means they secrete very little of the odour precursors secreted by most people of European and African descent [82]. The single nucleotide polymorphism (SNP) was first noted as it also leads to white earwax. Whatever the reason for the spread of the SNP, it seems that having less smelly armpits is not a disadvantage in finding a partner; perhaps these molecules are not important for sex [5]?

Second, while the apocrine glands, which provide most of the odorous secretions in the human armpit, may turn out to be involved, in most other mammals it is the sebaceous glands that are most important in chemical communication (and which differ most between species) [5,35, p. 126]. Perhaps sebaceous glands are the ones we should be researching. Sebaceous glands are found over most of the body, notably on the upper chest, back, scalp, face and forehead. The eyelids, the ear canals, the nostrils, the lips, the buccal mucosae, the breasts, the prepuce and the anogenital region all contain specialized sebaceous glands [36].

(c) The sampling and analytical techniques are ready

Recent advances in analytical techniques make the search for human pheromones more achievable than ever before. The search should use the well-established protocols established for sampling and identifying the pheromones and other molecules produced by other animals (e.g. [5,80,83]). We are simply another animal. Guidelines for good practice in reporting identifications of molecules in pheromone studies already exist, for example, in the Journal of Chemical Ecology [84].

(d) Methods for narrowing down the candidate molecules for bioassay

A major challenge when researching mammals is that secretions often include hundreds if not thousands of different molecules (figure 1) (e.g. human armpits analysed by Penn et al. [42]). One strategy to cope with this is illustrated by a study of male goat pheromones: since activity of the male scents was androgen-dependant, Murata et al. [16] looked for the molecules missing in the secretions of castrated male goats to find candidate molecules to test on females (figure 1). Only a systematic bioassay-led approach allowed Murata et al. [16] to identify the main pheromone component, 4-ethyloctanal. This molecule barely showed above the base-line at approximately 28:30 min in the top trace (figure 1) (compare upper and lower traces). Tens of other molecules on the upper trace had higher peaks.

For humans, we could compare, for example, differences in the molecules given off by adult males, adult females and children before puberty. The changes in the smells we give off as we become adults are due in large part to the development of sebaceous and apocrine skin glands at puberty, as in other mammals [5], including other primates [19,20].

Another potential approach to find candidate pheromone molecules in the distant future could be based on identifying the ligands for human olfactory receptors (ORs) [85] that are highly conserved (in contrast to most human ORs which are highly variable between individuals [86,87]). Highly conserved ORs might result from strong stabilizing selection against mutations reducing sensitivity in pheromone-detecting ORs [5]. (Incidentally, receptors involved in detecting androstenone and androstadienone are among the ORs that are highly variable between individuals [88].) However, to-date few human ORs have been characterized and only approximately 10% have been linked to their ligands [85,86] so such an approach is currently hypothetical. In addition, some ORs might be highly conserved because they are also expressed in other tissues (e.g. OR17–4 in sperm, [89]). In any event, we will still need bioassays of candidate molecules discovered in this or other ways.

(a) Funding for research into human olfaction and pheromones is a low priority

As well as its intrinsic scientific and human interest, if human pheromones were to be found this could open up new medical interventions and drug leads. A human mammary pheromone, if found, could be a lead for an externally applied drug facilitating prompt initiation of suckling by newborn babies (crucial for their healthy survival, [93]). If reproductive effects of human pheromones were to be found, such as the controversial phenomenon of menstrual synchrony (above), then this could provide leads for new concepts in contraceptive drugs, for example.

However, human olfaction is not seen as a health funding priority, even though losing your sense of smell significantly reduces the quality of life [96–98]. An indication of society's relative concerns is perhaps given by the founding dates of United Kingdom's organizations for blind people (1868), deaf people (1911), but not until 2012 for people affected by smell and taste disorders (Fifth Sense). Perhaps indicative of funding agencies worldwide, the UK Medical Research Council's priorities may be reflected by specific mentions of vision and hearing (but not smell) under its ‘sensory neuroscience research’ remit [99].

Research funding may have to specifically encourage serious study on human pheromones. The research is interdisciplinary by its nature and though good chemistry is essential, the molecules are unlikely to be exciting in themselves to chemists. Breakthroughs are unlikely to come within 3 year timescales common to grants. Instead we will need to provide long-term funding to get the subject moving.

I thank Dorothy Bishop, Victoria Braithwaite, Pau Carazo, Heather Eisthen, David Karlin, Sujai Kumar, Darren Logan, Robert Taylor, Joan Wyatt and Vivian Wyatt for valuable comments on drafts. I thank Yukari Takeuchi for kindly supplying figure 1. Two anonymous referees are gratefully thanked for their constructive comments on the manuscript.