Clustered holes, bumps, and similar patterns disgust some people.

Some 15%-17% of people may experience this disgust, called trypophobia.

Trypophobia is not considered a true phobia, though it's poorly understood.

Many researchers agree trypophobia has instinctive roots in the human brain, but disagree about its possible ties to fears from our evolutionary past.

Does a photo of a lotus fruit, below, make your skin crawl?

If you can't see it yet and think it might trigger you, now would be a good time to stop scrolling.

Previous research suggests as many as 18% of women and 11% of men — or 15% of the general population — become viscerally upset after looking at images of clustered holes or bumps, according to research on the condition colloquially known as trypophobia.

These clusters of holes are common in nature. They range from the creepy, like the back of a female surinam toad, to more mundane sights like honeycomb or clusters of soap bubbles.

A 2013 paper in the journal Psychological Science quotes how one sufferer feels when facing a triggering image: "[I] can't really face small, irregularly or asymmetrically placed holes, they make me like, throw up in my mouth, cry a little bit, and shake all over, deeply."

This image of a lotus seed pod may trigger disgust in people who report experiencing trypophobia. 3Point141/Wikipedia (CC BY-SA 4.0)

Though trypophobia is called a "fear of holes," the more researchers look into it the more they find it's not so much a fear, and not only of holes.

The phobia also isn't recognized by the psychological community as such. This is because it doesn't really have the signs of a true phobia, at least in the diagnosable sense.

"Trypophobia is more akin to disgust than to fear, and that the disgust is probably an overgeneralisation of a reaction to possible contaminants,"Arnold Wilkins, a psychologist at the University of Essex, previously told Tech Insider in an email. "The disgust arises from clusters of objects, and these objects are not necessarily holes, despite the name trypophobia."

It's a complex problem, and scientists like Wilkins continue to study, quantify, and try to explain trypophobia and its origins in the human mind.

Disgust rooted in survival?

Yum honey. Yuck holes. Wikipedia/Merdal Wilkins and his co-researcher Geoff Cole similarly think this strange revulsion could be rooted in biology, that we've evolved to fear these formations because when found in nature they are somehow dangerous.

To identify this effect, the researchers analyzed images found on trypophobia websites and images of holes that don't trigger trypophobia, looking for differences.

Then, when one of the self-reported trypophobics they interviewed mentioned a fear of the pattern on a blue-ringed octopus, they had what Cole has called a "bit of a Eureka moment," during which he realized a potential evolutionary reason for this fear of weirdly clustered holes — an association with a potentially poisonous or dangerous animal.

Here's the the blue-ringed octopus, which has venom powerful enough to kill a human:

The poisonous blue-ringed octopus. Jens Petersen To test their theory that those feared formations are associated with danger, the researchers collected 10 images of the top 10 poisonous species to analyze. The species they selected included the box jellyfish, the Brazilian wandering spider, the deathstalker scorpion, the inland taipan snake, the king cobra snake, and the stonefish and a few more, shown below.

They showed a puffer fish, whose liver and skin contain a poison. It's the second-most poisonous vertebrate in the world:

The toxic puffer fish. Jacopo Werther And a poison dart frog (which is, as its name implies, also poisonous):

A poison dart frog. public domain They also showed a marbled cone snail, whose sting can be fatal to humans:

Marbled cone snail. H. Zell The researchers analyzed their look and found that these poisonous species sometimes have a pattern similar to the ones that revolt trypophobes. They think ancient selection pressures on humans to avoid the types of patterns found on some poisonous animals and plants could have evolved into trypophobia.

"There may be an ancient evolutionary part of the brain telling people that they are looking at a poisonous animal," Cole said in a 2013 press release. Put another way: The disgust some feel may well provide an evolutionary advantage, even if unconscious, because it makes people with trypophobia want to run as far as possible from the holey-looking thing.

"We think that everyone has trypophobic tendencies even though they may not be aware of it," Cole said in the release. "We found that people who don't have the phobia still rate trypophobic images as less comfortable to look at than other images."

However, an April 2017 study in the journal Psychological Reports questions the assertion that venomous animal patterns and trypophobia share a connection.

The researchers, who were based in China, showed photos of venomous animals to 94 preschoolers as well as trypophobic images, and the correlation fell apart.

"[T]he discomfort felt toward trypophobic images might be an instinctive response to their visual characteristics rather than the result of a learned but nonconscious association with venomous animals," they wrote.

Instead, they added, it could be a more primitive feature of the mind that has no apparent explanation, akin to the sound of fingernails scratching a chalkboard.

Developing a trypophobic scale

Wilkins's graduate student An Trong Dinh Le, who himself has experienced intense trypophobia, has worked with Wilkins and Cole on their trypophobia research.

The team published a follow-up paper in 2014 in The Quarterly Journal of Experimental Psychology, in which they developed a scale to better measure people's reactions to these trypophobic images. The researchers also analyzed images that induce trypophobia to understand exactly what it is that causes the unpleasant reaction.

They discovered that trypophobia-inducing images contain some characteristics that differ from other images of nature, which are generally high-contrast (lots of brightness difference between big features) with low-contrast details (not a big difference in brightness between small features). When images don't have these natural features, they are generally more uncomfortable to view, Wilkins said.

For example, the image below and at right has a lot of contrast to reveal details. Knowing this, the researchers reduces how trypophobic an image was by reducing the contrast of its details.

Here's a sample of a filtered (left) and unfiltered (right) image that scores as trypophobic:

Filtered image is on the left, and the original trypophobic image is on the right. An Le/Tech Insider However, there are images with similar visual features that are associated with trypophobia, but — mysteriously — do not trigger a trypophobic response.

One example is the pattern of stripes on escalator stairs, which has unnatural spatial characteristics. It's unpleasant to look at and dazzles the eyes, but does not cause a trypophobic reaction, Wilkins said.

They also learned that trypophobes aren't just disgusted by clusters of holes — they respond just as negatively to clusters of bumps, as well.

"Given the large number of images associated with trypophobia, some of which do not contain clusters of holes but clusters of other objects, these results suggest that holes alone are unlikely to be the only cause for this condition," the researchers write in the paper. "We consider that the fear of holes does not accurately reflect the condition."

A different research group in Colorado has tried to better quantify trypophobic reactions by coming up with a scale to track the body's automatic responses. When someone with trypophobia looks at disgust-inducing images, their heart rates rose and their fingers began to sweat, according to an April 2017 study in the journal Personality and Individual Differences.

While that used only 37 college undergraduates, about 17% of them (roughly the same portion of people in Wilkins' 2013 study) displayed trypophobia-like fear.

"Although trypophobia at first glance seems irrational, these images might be triggering a primitive threat detection system," the authors wrote.

But what in our deep past as humans could have led to the development of a trypophobic triggering system? The jury's still out on that one — including if such a system exists at all, and if it has a role in our survival.

Jennifer Welsh wrote previous versions of this post, which we've updated to include new peer-reviewed research about trypophobia.