How the Smells of Our Hands Betray Our Gender

At the scenes of crimes or misdemeanors, fingerprints are of limited interest if they are not found in sufficient quantity. Hence the importance of looking for other clues. Why not smells? Varying according to age, diet or state of health, body perfume says a lot about our identity. Several studies have also suggested that these invisible traces differ according to our sex, without however demonstrating this association. From the fragrances emanating from the hands of several people, a team of American researchers from the University of Florida in the United States nevertheless succeeded in predicting their sex with almost perfect precision. “It could change the trajectory of how we currently use human scent in forensic science,” write Chantrell Frazier and his colleagues, chemists and study authors.

In this study published in PlosOne on July 5, scientists investigated whether the olfactory characteristics of hands were reliable enough to provide valuable information about the sex of their hosts. The idea is enticing because, in the context of police investigations for theft, assault or even rape, the hands are often involved... But then, where do the smells of our hands come from? When they are not related to the use of chemicals (cosmetics, disinfectant, soap etc.), food or hygiene, palm odors are attributable to the action of billions of bacteria that feed dead skin cells or even sweat that they degrade into volatile organic compounds (VOC), that is to say odors.

But that's not all. Our cells contain Major Histocompatibility Complex (MHC) molecules which are known for their role in immune responses but which are also referred to as 'odor carriers'. Several studies have shown in particular that women and men do not have the same CMH profiles and that, consequently, their olfactory signature is quite distinct. Hence the idea that we can discriminate between the sexes according to body odor: "Finally, the originality of this new study is to have confirmed this idea thanks to simple analyzes based on samples of hand odors of humans of both sexes", comments Olivier Delémont, professor of forensic science at the School of Criminal Sciences of the University of Lausanne.

To show this, the authors took the VOCs present on the palms of 30 men and 30 women by dabbing a gauze pad on their previously washed hands. This allowed them to get a full smell sample, stripped of food and cosmetics residue. They then isolated the odorous molecules from each sample using commonly used analysis techniques: gas chromatography, which makes it possible to separate volatile molecules contained in a mixture, and mass spectrometry, which makes it possible to identify them individually.

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All of the olfactory characteristics were then used to feed a modeling algorithm. “The authors somehow trained the algorithm to recognize that the bouquet of compounds X came from the sample Y, which was itself taken from a woman. By repeating the task for all 60 samples, the model was able to learn that such and such a combination of odorous compounds belonged to the female profile or the male profile,” explains Olivier Delémont. After teaching the model to recognize the samples, the authors put it to the test: they gave it unknown samples to find out if it was able to classify them in the male group or the female group. And the model managed to do it with a success rate of 96%.

That being said, can we really hope for a potential application of the tool in criminal sciences, as the authors suggest? "It's not won," says Professor Delémont. Like everything else, forensic science remains fallible due to sampling errors or data manipulation. But one of the main obstacles remains that the samples were taken under controlled conditions directly from the palms of the individuals, which were also very clean. “You can imagine that these conditions are far from the reality of a crime scene. Let's take the comparison with a party: all the guests have to touch a door handle several times, so that the smells present are in fact a mixture of different smells to which are added a lot of other residues", underlines the forensic science professor.

The second limit is that a person's odors vary over the course of a day, in particular depending on what one eats, the products applied and, more subtly, according to their state of stress. "Whether it's the victim or the aggressor, in a stressful situation, their smell will not be the same because their organism will react differently," explains Olivier Delémont. However, other studies have already shown that body odor, even partial, can give many other clues, such as age and ethnicity. The advantage would therefore be to be able to combine the analysis of different chemical fingerprints in order to use them when other investigative elements are lacking.