Are humans constantly but subconsciously smelling themselves?

Abstract

All primates, including humans, engage in self-face-touching at very high frequency. The functional purpose or antecedents of this behaviour remain unclear. In this hybrid review, we put forth the hypothesis that self-face-touching subserves self-smelling. We first review data implying that humans touch their faces at very high frequency. We then detail evidence from the one study that implicated an olfactory origin for this behaviour: This evidence consists of significantly increased nasal inhalation concurrent with self-face-touching, and predictable increases or decreases in self-face-touching as a function of subliminal odourant tainting. Although we speculate that self-smelling through self-face-touching is largely an unconscious act, we note that in addition, humans also consciously smell themselves at high frequency. To verify this added statement, we administered an online self-report questionnaire. Upon being asked, approximately 94% of approximately 400 respondents acknowledged engaging in smelling themselves. Paradoxically, we observe that although this very prevalent behaviour of self-smelling is of concern to individuals, especially to parents of children overtly exhibiting self-smelling, the behaviour has nearly no traction in the medical or psychological literature. We suggest psychological and cultural explanations for this paradox, and end in suggesting that human self-smelling become a formal topic of investigation in the study of human social olfaction. This article is part of the Theo Murphy meeting issue 'Olfactory communication in humans'.

Keywords: body-odour; face-touching; self-sampling; self-sniffing; social chemosignalling.

Figure 1.

Humans often bring a hand to their nose and sniff it. (a) A heat-map of face-touching in 160 participants. Colour reflects the proportion of the cohort that increased face-touches over baseline at that facial location (red = increase). Most face-touching is directly at or around the nose. (b) Measurement of nasal airflow during face-touching versus non-face-touching periods in the same session in 17 participants. Blue lines reflect participants that increased airflow and pink lines reflect participants that decreased airflow. Face-touching was accompanied by near doubling of nasal airflow, or in other words, a sniff. Adapted from [12]. (Online version in colour.)

Figure 2.

Illustration of typical postures that may serve subconscious hand-smelling. We reviewed the several hundred participants in our videoed experiments and identified numerous stereotypical postures for placing the hands at the nose. Given that these were often accompanied by a measurable sniff, we propose that these postures subserve subconscious hand-sniffing. This is a staged illustration figure, and the presented postures reflect a typical but not exhaustive set. (See also figure 10a.)

Figure 3.

Humans who may be sniffing their hands. A screen-shot from electronic supplementary material, video S1 ( also available at https://youtu.be/mKpOGdIVjDI), time: 00:40. In the edited video, we used orange circles to highlight individuals with a hand at their nose, and red circles to highlight those cases where you can literally ‘see’ the action of sniffing (although we think hand-sniffing is occurring in almost all of these cases, albeit less explicitly). The person to the lower left of the individual indicated with the red circle is not highlighted because his hand is not above his lower lip, thus not meeting our criteria for hand-sniffing face-touches. We further turn the reader’s attention to time point 00:52 in electronic supplementary material, video S1 for our favourite instance in this edited video, where despite the lack of any information (it is soundless), it appears that the red-highlighted person is self-consciously embarrassed by something that was said and appears to sniff her own hand in what we speculate might be reassurance. (Online version in colour.)

Figure 4.

Online self-report survey demographics. (a) Respondent proportions by sex. (b) Respondent proportions by age. (c) Respondent geographical distribution. Map by Freepik. (Online version in colour.)

Figure 5.

Humans self-report sniffing themselves and their conspecifics. Distribution of replies to all the questions asked in the questionnaire. The values reflect percentage (rounded) of respondents. M = men, W = women. For questions with a significant sex-difference in replies, each sex is represented separately. (Online version in colour.)

Figure 6.

Parents report increased hand-sniffing in children aged 3–6. (a) Nearly 40% of parents noticed a stage of increased hand-sniffing in their children. (b) The frequency of reported hand-sniffing in children by age. An increase was observed primarily between the ages of 3 and 6.

Figure 7.

Self-reported romantic partner-sniffing as a function of age. Whereas in the age-group 29–33, women reported significantly higher partner-sniffing than men, this pattern shifted such that in the age-group 41–74, men reported more partner-sniffing than women. Q1 n_F = 9, n_M = 30; Q2 n_F = 72, n_M = 30; Q3 n_F = 61, n_M = 29, Q4 n_F = 48, n_M = 40. We have no explanation for this pattern.

Figure 8.

Handshaking transfers volatiles from skin. (a) A representative image of our sampling method using a nitrile glove during handshake. (b) An example chromatogram from one experiment. Note that the ‘clean' condition is a glove worn by the same hand, but not shaken. The only three peaks that were present following all shakes but never once in the control are those we describe in the following panel. (c) Summated data from 10 individuals (each an average of three shakes) demonstrating three compounds of interest in chemosignalling (geranyl acetone (G), squalene (S) and hexadecanoic acid (H)) that were effectively transferred by handshaking in all instances and never once in the control. Error bars show standard error, **p < 0.01, ***p < 0.001. a.u., arbitrary units. Adapted from [12]. (Online version in colour.)

Figure 9.

Humans sniff their hands after handshaking. (a) Change in face-touching duration post-handshake within-sex in 66 women. Different conditions were either untainted, or tainted with odour (either a generic perfume, or one of the steroidal compounds 4,16-androstadien-3-one (AND) or oestra-1,3,5(10),16-tetraen-3-ol (EST)). Sniffing of the shaking hand (right) increased after handshake, increased even more in the presence of a perfume, but reversed in the presence of the steroids. A mirror-image pattern is evident in the left hand. *p < 0.05, **p < 0.01. Adapted from [12]. (b) Change in face-touching duration post-handshake within-sex in 18 men with autism (ASD) and 18 typically developed men (TD). Adapted from [22]. (Online version in colour.)

Figure 10.

It's not what you look at that matters, it's what you see. (a) An additional typical posture of placing the hand at the nose. (b) The same typical posture, here in Henri Vidal's 1896 sculpture of Cain after killing his brother Abel. (c) The same typical posture, here in a non-human primate. (d) The same typical posture, here witnessed in Sigmund Freud himself (1935). Although others may see in these images displacement behaviours, what we see in all four images are apes smelling the inside of their hand.