The smell of age

New findings from the Monell Center reveal that humans can identify the age of other humans based on differences in body odour. Much of this ability is based on the capacity to identify odours of elderly individuals and, contrary to popular supposition, the so-called ‘old-person smell’ is rated as less intense and less unpleasant than body odours of middle-aged and young individuals.

“Similar to other animals, humans can extract signals from body odours that allow us to identify biological age, avoid sick individuals, pick a suitable partner and distinguish kin from non-kin,” said senior author Johan Lundström, a sensory neuroscientist at Monell.

Like non-human animals, human body odours contain a rich array of chemical components that can transmit various types of social information. The perceptual characteristics of these odours are reported to change across the lifespan, as are concentrations of the underlying chemicals.

Scientists theorise that age-related odours may help animals select suitable mates: older males might be desirable because they contribute genes that enable offspring to live longer, while older females might be avoided because their reproductive systems are more fragile.

In humans, a unique ‘old person smell’ is recognised across cultures. This phenomenon is so acknowledged in Japan that there is a special word to describe this odour: kareishū.

Because studies with non-human animals at Monell and other institutions have demonstrated the ability to identify age via body odour, Lundström’s team examined whether humans are able to do the same.

In the study, published in the journal PLoS ONE, body odours were collected from three age groups, with 12-16 individuals in each group: young (20-30 years old), middle-age (45-55) and old-age (75-95). Each donor slept for five nights in unscented t-shirts containing underarm pads, which were then cut into quadrants and placed in glass jars.

Odours were assessed by 41 young (20-30 years old) evaluators, who were given two body odour glass jars in nine combinations and asked to identify which came from the older donors. Evaluators also rated the intensity and pleasantness of each odour. Finally, evaluators were asked to estimate the donor’s age for each odour sample.

Evaluators were able to discriminate the three donor age categories based on odour cues. Statistical analyses revealed that odours from the old-age group were driving the ability to differentiate age. Interestingly, evaluators rated body odours from the old-age group as less intense and less unpleasant than odours from the other two age groups.

“Elderly people have a discernible underarm odour that younger people consider to be fairly neutral and not very unpleasant,” said Lundström. “This was surprising given the popular conception of old-age odour as disagreeable. However, it is possible that other sources of body odours, such as skin or breath, may have different qualities.”

Future studies will both attempt to identify the underlying biomarkers that evaluators use to identify age-related odours and also determine how the brain is able to identify and evaluate this information.