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‘Old-person smell’ is a real thing — and it’s not just due to mothballs or a musty house. Researchers at the Monell Chemical Senses Center in Philadelphia confirmed that elderly people really do have a distinct scent, so recognizable in fact that people can identify them by body odor alone.

The researchers speculate that the human ability to discern age through scent might be an evolutionary skill related to the way other animals are able to sniff out young, virile mates and avoid those that are older or sick.

Both human and non-human animal body odors are rich with chemical components that can transmit useful social information, the scientists say. And many animals, including mice, rabbits, owls and monkeys, are known to undergo changes to the chemical composition of their body odor as they age.

“Similar to other animals, humans can extract signals from body odors 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, in a statement.

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Lundström became interested in studying the effect of age on human body odor when he noticed that old people in the U.S. seemed to smell just the way did back home in Sweden. One day, when walked into an elderly care center near Philadelphia to give a lecture, he realized that the smell of the place was familiar — it was the exact same scent of the nursing home in Sweden where his mother worked when he was a boy.

For Lundström’s study, researchers collected body odor samples from 44 volunteers of three different age groups: 20 to 30 years old (young), 45 to 55 (middle-aged), and 75 to 95 (old). Researchers acquired the samples by sewing nursing pads into the armpits of unscented T-shirts and asking each body odor donor to sleep in the same shirt for five consecutive nights. The pads were then removed, cut into quadrants and placed into individual glass jars.

Next, 41 young (20 to 30 years old) participants were given two glass jars in nine combinations to evaluate the scents. The participants were asked to rate the intensity and pleasantness of each odor; they were also asked either to identify which sample came from the older odor donor or to estimate the age group to which each sample belonged.

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Participants were able to single out old-person smell better than the odors of the younger age groups. When people tried to categorize the sample odors by age, they had trouble distinguishing between the young and middle-aged, but they were often instantly able to identify old people’s scents.

Interestingly, despite its uniqueness — and contrary to the stereotype — old-person smell isn’t exactly bad. The participants rated it as less intense and less unpleasant than body odor from younger donors. By gender, participants rated middle-aged men’s smell as the yuckiest and most intense, and old-man smell as the most pleasant and least intense. By contrast, middle-aged women’s scents were rated as more pleasant than old-lady smells.

“Elderly people have a discernible underarm odor that younger people consider to be fairly neutral and not very unpleasant,” said Lundström in the statement. “However, it is possible that other sources of body odors, such as skin or breath, may have different qualities.”

The old-person smell detected in the study couldn’t be explained by the donors’ lifestyle or environment. The donors were all instructed to wash with unscented soap and shampoo before bed each night. They were also asked to refrain from drinking alcohol, smoking and eating spicy foods or any other type of food that could affect body odor.

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The biological explanation for old-person smell isn’t clear, but the researchers note that human body odors “originate from a complex interaction between skin gland secretions and bacterial activity, and skin gland composition and secretion change in an age-dependent manner throughout development.”

Why the younger volunteers found old-person smell so unoffensive isn’t clear either, but the authors write:

In the non-human animal literature, the ‘good genes’ model has been put forth as an explanation for why female animals are attracted to the odors of older males or why female insects prefer the sex pheromone from older male insects. Signals indicating old age, supposedly regulated by the immune system, are favored due to the likelihood that individuals who reach old age possess a strong and adaptive immune system, as well as other adaptive advantages that have allowed them to grow older than their peers. Indeed, older male insects have a higher reproductive success than their younger competitors.

Of course, unlike insects, humans use many other better ways to choose among potential partners. Still, our ability to sniff out old folks appears to remain intact. “At the moment, the most interesting implication of this study is that this might be another signal that we can extract and use from body odors. We now know that we can extract information about the senders genetic composition, kinship, sickness and age,” says Lundström.

The study, published in the journal PLoS ONE, is part of a larger project that aims to identify what signals are communicated via body odors, isolate how the human brain processes these signals and determine how they alter our behavior in everyday situations.