Phobias are, by definition, irrational. But trypophobia, a seemingly weird fear described for the first time by British scientists this week, may actually have an evolutionary basis.

Trypophobia is an intense aversion to the sight of objects that are pockmarked or clustered with holes. Sufferers can be set off by things as innocuous as the seed pod of a lotus or a bar of aerated chocolate.

Gagging at flowers and chocolate probably doesn’t sound like a clever survival instinct. But almost by accident, Geoff Cole, a vision and attention expert at the University of Essex, discovered that trypophobic images share visual characteristics with highly poisonous animals.

“When a trypophobic individual looks at an image, there’s a part of their brain — an old evolutionary part of their brain — that’s trying to tell them, ‘Be careful,’ ” says Cole, lead author on the Psychological Science paper that documents trypophobia.

The fear of holes, while unusual and until now, undocumented, appears to be relatively common. Cole’s research showed that 16 per cent of 286 test subjects experienced revulsion when shown a trypophobic image.

Cole traces his own interest in the subject back to a day in metalworking class when he was 13. Another student drilled a series of holes into a coin and showed it to him.

“I felt a bit nauseous and had to sit down,” Cole says.

Decades later, after similar incidents, he decided to search out others who had the same reaction. After some googling, Cole found there was a significant community online that had coined the term “trypophobia” (after the Greek word for drilling or boring holes). Yet there was nothing in the scientific literature on the phenomenon, he found.

Cole approached his colleague Arnold Wilkins, a visual stress expert. They decided to investigate.

First, Cole analyzed trypophobic images others had collected online and discovered they shared a common visual trait.

They all had high-contrast energy at mid-range spatial frequencies: a technical way of saying that when held at arm’s length, the images all went from high contrast to low contrast every three centimetres or so. (A zebra’s coat has the same effect. While zebras are striped instead of spotted, Wilkins has previously reported that similarly patterned images induce migraines in some people.)

By chance, a trypophobe came to visit Cole in his office. The visitor mentioned that an image of an octopus had triggered intense revulsion. Cole immediately searched online to find out what it looked like, and discovered that the animal — a blue-ringed octopus — is one of the most poisonous in the world.

“That was a bit of a Eureka moment,” Cole said. He then searched for images of other highly poisonous animals and showed them to his visitor. The trypophobe couldn’t bear to look.

When Cole analyzed images of the poisonous animals, many of them shared the same unusual spectral qualities as the trypophobic images.

To test the general public’s aversion to these images, Cole and Wilkins gathered 20 non-trypophobic test subjects and showed them 50 trigger images and 50 normal ones. The students were asked to rate their discomfort on a scale of -5 to 5.

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The trigger images scored an average of -0.42 and the non-trigger ones 0.53 — in other words, it appears that almost everyone is slightly averse.

This suggests, Cole says, that trypophobia has an evolutionary basis. The researchers plan to test that hypothesis more rigorously in the coming years.