A Hidden Sense of Smell and the Sixth Sense

January 15, 2016

Noam Sobel can’t stop watching you sniff your hands. In public, he sees people doing it everywhere. Here a man is leaning on one elbow, his palm covering his nose. Over there someone is playing with her upper lip, her fingers hovering just below her nostrils. Maybe you’re doing it right now.

Don’t be embarrassed. We humans are animals, and a growing body of research suggests that like other animals, we use our sense of smell to gather information about those around us. Sobel, a Professor of Neurobiology at the Weizmann Institute of Science in Israel, thinks sniffing our hands might be just one way that we sample each other’s odours so that we can learn from them.

Findings from other researchers suggest that a second language, spoken in scent, might be passing (as it were) right under our noses. Body odours are in the background of all our interactions. And the clues about relationships and emotions that we sniff out from these odours might be a crucial element of human society.

The bodily smells of our fellow humans help us to recognise our relatives and friends and to bond with our babies. Even wearing a literal blindfold, we can recognise the musty aroma of an elderly person – an odour, by the way, that is not necessarily unpleasant. Meanwhile, the odour of illness can warn us to stay away from a contagious other.

Conventional wisdom holds that humans are not that gifted in the olfactory department. Watching our dogs and cats plant scent markings or sniff the air knowingly, we might feel as though our species is wearing a scent blindfold. Researchers trying to explain human sexual attraction and other interpersonal instincts couldn’t even find human pheromones – special molecules released by one individual and received by another of the same species, causing a specific reaction.

Ants use pheromones to communicate and leave trails back to the home base. Boar pheromones, when sprayed into a pig sty, will makae females that are in heat assume a mating stance. Yet, perhaps to our advantage in terms of conscious self-control, studies suggest that humans are happily immune. Sure, we can distinguish the smell of someone who has been drinking, or someone who ate garlic or curry recently – but a broad detection of such blatant or toxic odours seems a scant gift compared with species that can smell their way home.

Still, we do have a sense of smell, and surely it must be for more than just noticing that it’s time to take out the trash. That’s the thought that led Sobel to consider the olfactory significance of the handshake. It’s common in cultures around the world, if not quite universal.

Historians might explain the origin of handshaking with a story describing how to show a stranger that you’re not holding a weapon. But Sobel wondered whether there could be a deeper motive, too – something more like a hearty butt-sniff between dogs.

To test the idea, Sobel and his colleagues brought subjects into a room and left them alone while secretly filming them. After a few minutes, an experimenter came in and greeted each subject, either with a handshake or not, then left the room for another few minutes. Even before the team analysed the experimental results, the videos were striking.

‘People are constantly sniffing their own hands,’ Sobel said. Sitting in a room alone, subjects spent 22 per cent of the time with one hand or the other near their nose. We might think of this behaviour as nervous grooming – scratching, biting a nail – but when the researchers measured air flow through the nose on another group of subjects, they showed that people with their hands near their noses were actively sniffing. The paper appeared in 2015 in eLife.

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