Sexual incentive properties of odors

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While chemicals certainly possess unconditioned sexual incentive properties in non-human mammals, the existence of unconditioned, chemical sexual incentives in the human is uncertain. Similarly, the existence of conditioned olfactory stimuli with sexual incentive properties in the human is unknown. While we know that visual or auditory stimuli with sexual content enhance genital blood flow, there is no published study showing that an olfactory stimulus can have this effect. The closest approximation is a study where young women, as always undergraduates, were asked to read either an explicit account of a sexual encounter between a man and a woman on a train or a story about a French journalist. There was no sexual content in the story about the journalist. Half of the subjects were invited to wear a mask impregnated with 300 \ig of androstenol and the other half wore a control mask. The mask covered the mouth and the nose, assuring that the steroid reached the olfactory epithelium. After having finished reading, the subjects answered a questionnaire evaluating sexual arousal and some other mental events. Results showed that androstenol had no effect on self-rated sexual arousal independently of the reading material provided. Reading the pornographic text produced, as expected, a higher level of arousal than the neutral text. The conclusion was that androstenol did not influence sexual feelings (Benton and Wastell, 1986). An interesting aspect of this study was that the chemical may have come in contact with the vomeronasal organ, if it had existed.

In another study, 39 young men and 39 young women, all undergraduates at a Canadian university, watched a 15 min slide show together with an opposite sex confederate. The confederate wore either a few drops of an androstenol solution applied on each wrist and behind each ear, or a synthetic musk applied on the same places, or no odor. After the slide show the experimental subjects were asked to answer a questionnaire and to rate the attractiveness of the partner. Finally, the subjects were asked about the purpose of the study and then subjected to an odor test. None of the subjects was aware of the real purpose and all subjects included in the analyses were able to detect the odor of androstenol. Data showed that androstenol as well as musk failed to affect the ratings of the partner's attractiveness. The conclusion of this interesting study was clear-cut: 'The failure to find positive effects under these circumstances suggests that it is premature to classify androstenol as a human pheromone, the enthusiasm of the perfume industry notwithstanding' (Black and Biron, 1982, p. 329).

The lack of effect of androstenol was confirmed in another quite ingenious approximation to a test of sexual incentive motivation. Groups of homo- and heterosexual young men and women were asked either to arouse themselves with self-produced sexual fantasies or to enhance their appetite with self-chosen alimentary fantasies. The efficiency of the fantasies was determined by recording heart rate rather than genital blood flow, but heart rate is well known to increase during sexual arousal. Indeed, the sexual fantasies enhanced heart rate while the alimentary fantasies failed to do so. After the induction of sexual or alimentary arousal, the subjects were asked to wait for a few minutes in a waiting room with several chairs. One of the chairs had been discretely scented with androstenone. The chair chosen for sitting on during the waiting period was recorded. It was reasoned that if androstenone had sexual incentive properties, then the subjects would chose the scented chair more than the others, particularly since their sexual arousal previously had been enhanced by the fantasies. The data revealed that the androstenone chair was not chosen more than the other chairs regardless of whether the subjects were sexually aroused or not (Pause, 2004). Again, the conclusion must be that the steroid does not have any sexual incentive properties. Most humans are able to smell androstenol and androstenone (Gower and Ruparelia, 1993), so the failure to find an effect cannot be due to sensory incapacity.

The three studies described above are the only ones I know of that have tested the hypothesis that an olfactory stimulus has sexual incentive properties in the human employing procedures that afford some scientific credibility. For the sake of completeness, I will mention a fourth study, although it does not offer much useful information. However, it can be employed as an example of the convoluted ways in which some enthusiasts design their experiments. As usual, undergraduate students in psychology or biology were used as subjects. There were 76 of them, 38 men and 38 women. They were asked to wear a perforated necklace containing a cotton swab soaked in an androstenol solution from the afternoon of one day until 9.30 in the morning of the next day. When showing up at 9.30 to return the necklace, the students were invited to answer a questionnaire and to fill out a diagram consisting of three concentric circles. The central circle depicted the subjects and the second one depicted others. The space between this second and the third circles was divided into segments and each segmented was labeled with some category of relationship to others, like relatives, friends, fellow students etc. This appears to have had some decorative function, because these relationship categories were never mentioned again. The subjects were asked to recall all verbal interactions they had had since waking up that morning and mark them with an arrow, the head pointing to the one that initiated the interaction. It turned out that women wearing the androstenol necklace initiated more verbal exchanges with men than women wearing the control necklace. There was no necklace-effect in men. The results were interpreted as showing that women exposed to the androstenol solution were more responsive to men than women in the control group (Cowley and Brooksbank, 1991). Whether the number of verbal exchanges early in the morning has some sexual significance or not is impossible to determine. Just to stimulate curiosity, I mention that the rationale for the authors' conclusion will become explicit in a few paragraphs.

There are many additional curious studies of the influence of chemicals on human behavior and even on sexual functions, but none of them is of any relevance to the problem of olfactory sexual incentives, or the role of pheromones for human sexuality. Nevertheless, I will mention one of these studies as a kind of entertainment. The scientists spent five weeks observing the choice of stalls in a rest-room for men and in another for women. Some weeks half of the stalls were treated with a solution containing androstenol and the other half, the control stalls, were treated with androsterone, a steroid without known pheromonal activity. Compared to baseline weeks, men avoided the androstenol-treated stalls during treatment weeks. Women did not show any change in choice between control and treatment weeks. These data were interpreted as showing that androstenol acts as a spacing pheromone in men (Gustavson et al., 1987). Since the subjects evidently avoided the androstenol treated stalls, androstenol must be a negative incentive, at least according to the concepts of incentive motivational theory. Although this fascinating rest-room study is of no direct relevance for sexual behavior, its results make it difficult to imagine that androstenol would be a sexual incentive.

Some comments must be made on the choices of olfactory stimuli employed in the studies mentioned and in the many not worth mentioning. We have already seen that the androgenic steroid androstenol and its derivatives are popular. The reason for this is that odors produced by boars are known to enhance the procep-tive behaviors of sows. The abundant saliva excreted by sexually excited boars was suspected to contain the relevant odor and it was soon found out that two steroids, produced in the testes and excreted by the submaxillary glands, androstenol and androstenone, were the critical molecules (Melrose et al., 1971; Signoret, 1976). The capacity of these compounds to stimulate pig sex has been confirmed on many occasions and they are even commercially available in the form of an aerosol spray named Boar Mate (Knight, 1985). So far, neither androstenol nor androstenone has been shown to have similar effects in any other species. The fact that some scientists believe that humans are more similar to pigs than to other mammals is not easy to explain. Nevertheless, the belief in some communities between human and porcine behavior is nicely illustrated in the Cowley and Brooksbank (1991) androstenol study. After having found that androstenol enhanced women's verbal exchanges with men, they most reasonably concluded that this effect was caused by an increased responsiveness to males. In the following paragraph, they remark that androstenol is known to rouse estrus sows to be sexually receptive and they continue by suggesting that there is much to be gained from a comparative approach. In fact, their study was based on the notion that data from sows could be immediately generalized to women. Whether this is to be taken as a compliment to the fair sex is uncertain.

A favorite odor source in human pheromone studies is sweat collected from the armpits. We can probably all agree that the odor produced by non-deodorized armpits is strong and salient, but everyday experience insinuates that it is not particularly attractive to potential sex partners. Nevertheless, there are those who have tried to show that armpit sweat is a positive sexual incentive. A concoction with a composition purportedly similar to underarm secretions was provided to 36 women. They were instructed to apply the concoction under the nose, on the cheeks and behind the ears at least once every other day for 6 weeks. Subjects recorded their sexual activities in a diary and they were also interviewed a few times during the study. The concoction was found to increase the frequency of sexual intercourse and formal dating. The authors concluded that the concoction acted as a sex attractant, enhancing the women's sexual attractiveness (McCoy and Pitino, 2002). In the language of incentive motivational theory, this means that the concoction had sexual incentive properties. A similar study, this time using post-menopausal women (mean age was 57 years) as subjects, reported that the same concoction enhanced the frequency of petting and kissing. No effect on the frequency of intercourse was found in these older women. Again it was concluded that the concoction was a sex-attractant (Rako and Friebely, 2004). Another study, performed in young men, showed that another concoction prepared specially for men, this time employed as aftershave, enhanced the frequency of sexual intercourse and of 'sleeping next to a romantic partner'. It was concluded that the concoction increased the sexual attractiveness of men (Cutler et al., 1998). This means that it functioned as a sexual incentive. It might seem, then, that chemicals similar to those found in the armpits are sexual incentives both for men and women. These data are not unproblematic, though. First, a re-examination of the data from the McCoy and Pitino (2002) and the Cutler et al. (1998) studies revealed that there was no increase in any sociosexual behavior in the groups carrying the concoction in either of the studies. The significances reported in the original reports were due to changes in the behavior of the control groups. Thus, 'the claim in both studies that the action of pheromones is through an increase in attractiveness to the opposite sex depends on unconvincing assumptions and is at best premature and receives no support in data' (Winman, 2004, p. 701). Second, the composition of the underarm secretions produced by men and women has been shown to be similar (Zeng et al., 1996). If these secretions are chemically similar, then their perceived odor should also be similar. Men and women should, then, produce exactly the same 'sex attractant' and men should be attracted to other men just as much as to women and women should be attracted to other women just as much as they are attracted to men. A third problem comes from the fact that humans invest considerable resources in getting rid of armpit odors. In the late 1980s, Americans alone spent 1.6 billion dollars per year on products to eliminate or mask such odors (Ramirez, 1990) and that figure has probably not been reduced since then. If axillary odors indeed enhanced our attractiveness, there should be no reason for getting rid of them. Particularly not considering that we also invest huge sums in perfumes and other liquids with the purpose of enhancing our odorous attractiveness.

Almost absent from human studies of chemicals as sexual incentives are the only group of compounds that could be predicted to have that function. A long time ago it was found that vaginal secretions from women contain a number of aliphatic fatty acids such as propanoic, methylpropanoic, butanoic, methylbu-tanoic and methylpentanoic acid (Michael et al., 1974). Most interestingly, the concentrations of these volatile fatty acids had their maximum during the late follicular phase (Michael et al., 1975). Furthermore, the concentrations of them were rather similar to what had earlier been reported for several primate species (Michael et al., 1972). The attentive reader may also remember that these acids have been found to enhance sexual arousal and copulatory behavior in nonhuman primates (see Chapter 3). In my humble judgment, it is surprising that scientists have tested chemicals working in boars but in no other mammal and stinking armpit products that most humans find detestable instead of carefully evaluating the volatile fatty acids for which some data from primates do exist. One possible reason for this is that the two studies performed in humans offered negative results. In the necklace study mentioned above (Cowley and Brooksbank, 1991), a group wearing a necklace containing the fatty acid mixture typical of the human vagina was included. Neither men nor women reported any change in verbal exchanges as a result of walking around with the necklace. This is not surprising, however. First, there is no reason to believe that heterosexual females should be aroused by typically female odors. Second, there is no reason to believe that the presence of these volatile fatty acids ever was perceived by the men interacting with the women in the study. Verbal exchanges, particularly in the morning, occur with some distance between the participants (the exception being those sharing a bed, but they were probably not many in the study), making it extremely unlikely that the concentration of the airborne fatty acids was above detection limit in the men's nose. Third, we do not know if the number of verbal exchanges with women is the most sensitive indicator of sexual incentive motivation in men. Thus, I conclude that the Cowley and Brooksbank (1991) study is inconclusive.

More conclusive data were obtained in a direct evaluation of the effects of vaginal odors on self-reported sexual motivation and frequency of intercourse. Sixty-two young couples were paid $1.00 per day for participating in the study. Perhaps that sum had some significance back in 1978. The women's task was to apply a mixture of aliphatic fatty acids similar to the one found in the vagina of women and apes to the skin of the chest at bedtime. Control odors were alcohol (the solvent used for the fatty acid preparation), water and a perfume called Heather. The women did not know which scent they applied. The participants were asked to keep a record of the time of scent application and the possible occurrence of intercourse. Thirty-four couples also answered a questionnaire purportedly evaluating sexual motivation, sex play and the occurrence of orgasm. There was no effect of vaginal odors or of any other scent on any parameter (Morris and Udry, 1978). This apparently well-controlled study strongly suggests that vaginal secretions do not function as sexual incentives in the human.

I mentioned in Chapter 3 that a biologically irrelevant novel odor stimulated sexual arousal and copulatory behaviors in male monkeys as much as vaginal secretions did. An entertaining experiment evaluated the possible effects of this kind of odor on the attractiveness of women. Male psychology undergraduates were told that they participated in a study of factors determining the 'first impression of others'. Each male was paired with a female confederate and subjected to an interview during which the subject was sitting side by side with the confederate. Completely non-controversial questions like, 'What are your favorite leisure time activities?' were asked and the confederate was instructed to give equally non-controversial answers according to a memorized script. After the interview, the subject (and the confederate) was asked to answer a questionnaire evaluating the attractiveness of the partner. Some confederates wore a perfume with the poetic name of Jungle Gardenia and others wore no scent. Other psychology students had rated the perfume as pleasant and olfactory potent. Some of the confederates were neatly dressed in blouse, skirt and hose while others were dressed in the typical student uniform, jeans and a sweatshirt. Results show that the scent made the informally dressed women appear more 'romantic' than the unscented. The opposite was true for the neatly dressed women. Regarding attractiveness, the only effect obtained was an interaction between scent and dress in the way that informally dressed women were more attractive if smelling of perfume, while the neatly dressed women were less attractive when perfumed. My conclusion is that you had better smell good if you are lousily dressed and better avoid it if your dress is cute. The author prefers a more conservative conclusion, namely:

while pleasant aromas sometimes exert positive effects on social behavior, they do not always do so. Thus, contrary to what advertisements often suggest, unquestioned faith in the benefits of perfume, cologne, and similar products does not seem justified (Baron, 1981, p. 616).

I find no reason to disagree. This study illustrates quite neatly that non-natural odors may enhance attractiveness, or they may reduce attractiveness, depending on circumstances. Results from this study are just as unconvincing as those of the studies mentioned in the preceding paragraphs.

The preceding brief presentation of some data pertaining to the question of olfactory sexual incentives in the human should have made it clear that there is no evidence for any functionally important role for olfaction neither in sexual arousal nor in processes related to approach to a potential mate, or copulation for that matter. If we were extremely open minded and assumed, without having any good reason for it, that attractiveness is equivalent to sexual incentive properties, then we would have to conclude that either the role of olfactory stimuli for these properties in the human is so complex that it escapes our comprehension or that such stimuli are unimportant. My personal inclination for parsimony makes me stick to the latter alternative.

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