Thousands of years ago, our ancestors were already using latrines and tidying their hair with combs, suggesting we have some deep-rooted tidy tendencies. Yet people today still engage in gross habits, such as eating lunch over a keyboard or failing to wash their hands after visiting the loo.

The reason for these contradictions is that our natural inclination for cleanliness and good hygiene isn’t borne from reason, but driven by our sense of disgust. This emotion protects us from the risk of infection, but it’s far from foolproof or logical – it’s triggered by certain sights, smells and beliefs, rather than any objective measure of hygiene. Generally speaking, people are more bothered by dirt they can see and smell, even if it’s harmless, rather than germs that are invisible, even if more deadly.

The evolutionary roots of our instinct for cleanliness – as a way to protect ourselves from disease – explains other paradoxes. We might wipe down our own kitchen surfaces with antibacterial cleaner, yet collectively we fill the oceans
with plastic.

Looking across the animal kingdom, we’ve no reason to be smug. Not only do other creatures also show an inclination for being clean (birds keep nests clear of faecal matter, and bees remove their dead from the hive), but often they outdo us. Research by ecologists at North Carolina State University showed that, while human beds are filled with bacteria derived from our own bodies, this was much less the case with chimpanzees – probably because they go to the trouble of making a new treetop bed each night.

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Dr Christian Jarrett is a cognitive neuroscientist, science writer and author. He is the Editor of Psyche, the sister magazine to Aeon that illuminates the human condition through psychology, philosophy and the arts. Jarrett also created the British Psychological Society's Research Digest blog and was the first ever staff journalist on the Society's magazine, The Psychologist. He is author of Great Myths of The Brain and Be Who You Want: Unlocking the Science of Personality Change.