The Age-Old "Grandmother Rule" Is Backed By Science

Scientists investigated what they call "The Grandmother Hypothesis" when it comes to washing your skin, finding that people tend to miss several key areas, and their microbiome is less healthy as a result.

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The team from the George Washington University Computational Biology Institute was investigating the skin microbiome – that is, the community of microorganisms living on people's skin – of healthy individuals, with a particular focus on variations across skin areas, such as between the arms and those neglected areas of washing like the navel and behind the ears.

Keith Crandall, Director of the Computational Biology Institute and professor of biostatistics and bioinformatics at George Washington University, said in a 2023 statement that his grandmother had always told him to “scrub behind the ears, between the toes and in the belly button". He speculated that these areas may harbor different types of bacteria to other areas of the body, due to neglect during washing.

In the study, 129 students collected samples from their calves and forearms, behind their ears, between their toes and in their navels. These same students were taught to sequence the DNA in these skin samples, comparing the oily, neglected areas to the samples taken from dry, cleaner areas.

Sure enough, Crandall's grandma and Crandall were right; areas that were more regularly cleaned had a much more diverse microbiome, containing a potentially healthier collection of microbes than the neglected areas. 

"Dry skin regions (forearms and calves) were more even, richer, and functionally distinct than sebaceous (behind ears) and moist (belly button and between toes) regions," the team wrote in their study, adding that there were no significant differences found across genders, ages, and ethnicities. 

"Within skin regions, bacterial alpha- and beta-diversity also varied significantly for some of the years compared, suggesting that skin bacterial stability may be region and subject dependent," they added.

Your skin microbiome is made up of microbes that are helpful and harmful to you. When the balance shifts towards harmful microbes, it can result in diseases such as eczema or acne, Crandall said. However, the relationship between microbiome health and human health is still very much an area in need of investigation, with this study serving as a reference point for healthy microbiomes in adults.

An earlier version of this article was published in November 2023.