When psychologist Johan Lundstrom decided to test whether there really was an "old people smell" he got good news and bad news. Unfortunately, he found, people can tell the difference between young, middle-aged and old peoples' body odors. The good news is people aren't very good at telling which odors came from which age group – they could only tell they were different. Also in the bright side: women smell no worse as they age and men get nicer smelling after 75.

Lundstrom, who works at the University of Pennsylvania and the Monell Chemical Senses Center, said he decided to investigate this matter when giving a recent talk at a retirement home near Philadelphia. He smelled something familiar that took him back to his childhood. "As soon as I stepped in the home I recognized an odor that I'd smelled in Sweden when my mother was working as a nurse in a retirement home," he said. How, he wondered, could you have the same odor in two different populations on two different continents?

At first, he did what most of the rest of us do – he used Google. That turned up some anecdotal reports about old-age smell, and the fact that the Japanese even have a word for it – kareishu.

Then he set up his own body-odor experiment in collaboration with colleagues at Penn, Monell and the Karolinska Institute in Sweden. The results were published in Friday's issue of the journal Public Library of Science One.

He recruited a group of "donors" and a group of 41 volunteers willing to sniff them. The donors were chosen to fit into three discreet groups: young(20-30), middle aged(45-55), and old(75-95). To keep the sniffers blind to the actual ages of the participants, he collected the odors of the "donors" using t-shirts with special pads sown into the armpits. Some studies involve donors that run on treadmills, he said, but the smell of exercise-induced sweat is different from ordinary body odor. The odors originate from different glands and contain different chemicals.

He asked the donors to wear the shirts to bed after showering and washing with a non-perfumed soap. Then they slept in the shirts for five nights in a row – enough to catch some genuine body odors.

The sniffers had to perform several different tasks. First they had to figure out which odors were similar to each other and place all the similar ones in groups. This they did quite well, said Lundstrom. They did a particularly good job of grouping all the odors from the old people, and were slightly less adept at distinguishing middle-aged b.o. from youthful b.o.

So we humans seem to be able to distinguish odors from different aged donors as different, but a second task showed we're not so good at tying those differences to age categories. For that task, the sniffers were asked to try to categorize the different odors by age after Lundstrom pooled the odoriferous pads from each group. The sniffers performed better than chance when it came to identifying the pooled donors in the 75-95 category, said Lundstrom, but not much better. They weren't able to tell which of the other two pooled odors came from young or middle-aged donors.

Lundstrom said several studies have shown that mice and other mammals can use scent to estimate age among members of their own species. One study he came across showed that people could distinguish the ages of other animals.

One possible explanation, he said, is that animals are tuned to sniff out age because they benefit from mating with older individuals. In many animals, the older ones have proven survival ability, and so potential mates can benefit from recognizing this as attractive. Old animal smell is a sign of good genes.

Humans are unusual because we have menopause and older women lose fertility, but this is not the case in many other animals. In chimps, for example, females continue to reproduce until they die, and they get more attractive to males the older they get. Anthropologists studying wild chimpanzees in Uganda report that males find the grandma females the sexiest of all.

The other possibility, Lundstrom said, was that the sniffers were detecting the smell of inflammation and creeping illness. Change inside the body might be altering the odor outside. One possibility he raised in the paper was that a type of white blood cell called polymorphonuclear leukocytes change as we age and could also alter our body odors. But that's still not known.

If the smell changes are from decay, at least they weren't rated as unpleasant. In men it was quite the opposite, said Lundstrom. "Young guys are stinky, middle aged guys are even more stinky, and when they got old it goes away."

This, he sees as a sort of second childhood of body odor. The concentration of odor-producing chemicals declines back to pre-pubescent levels as men get old. "They return to childhood levels by age 80."

Contact Faye Flam at fflam@phillynews.com;/[URL or @fayeflam on Twitter. Read her blog at philly.com/evolution.