A deep-seated anxiety about parasites and infectious diseases may be behind 'coffee bubble phobia' - an intense aversion some people feel when they see clusters of roughly circular shapes, such as the bubbles in a cup of coffee or the holes in a sponge.
Previous explanations for the condition include the suggestion that people are evolutionarily predisposed to respond to clusters of round shapes because these shapes are also found on poisonous animals, like some snakes and the blue-ringed octopus.
However, psychologists at the University of Kent in the UK suggest that the condition - known as trypophobia - may instead be related to an evolutionary history of infectious disease and parasitism that leads to an exaggerated sensitivity to round shapes.
Many infectious diseases result in clusters of round shapes on the skin - smallpox, measles, rubella, typhus, scarlet fever etc.
Similarly, many ectoparasites, like scabies, tics and botfly also lead to clusters of round shapes on the skin.
Researchers recruited over 300 people from trypophobia support groups and 300 students without trypophobia who were invited to view sixteen cluster images.
Eight were pictures of clusters relating to diseased body parts (eg circular rash marks on a chest, smallpox scars on a hand, a cluster of ticks), and the other eight cluster images had no disease-relevant properties (eg drilled holes in a brick wall, a lotus flower seed pod).
Both groups of participants reported finding the disease-relevant cluster images unpleasant to look at.
However only the trypophobic group found the disease- irrelevant cluster images extremely unpleasant.
Individuals with trypophobia experience an overgeneralised response, to the extent that even an image of bubbles on a cup of coffee can trigger aversion in the same way as a cluster of tics or lesions.
Previous research has shown that the function of the emotion disgust is to motivate people to avoid sources of potential infection, so the researchers predicted that unlike most phobias (eg snakes, heights, dogs) which mainly involve intense fear, people with trypophobia would predominantly experience intense disgust.
They asked the individuals with trypophobia to describe their feelings when looking at cluster images.
Majority of individuals with trypophobia experienced disgust even towards the disease-irrelevant cluster images like a sponge or bubbles.
Only a small proportion described feeling fear or fear- related feelings.
Trypophobic individuals frequently reported feelings like skin itching, skin crawling or even the sensation of 'bugs infesting the skin'.
This skin response suggests that people with trypophobia may perceive cluster stimuli as if they are cues to ectoparasites, even leading some to feel as if they are infested.
The findings showed that although trypophobia has been described as the 'fear of holes', it would be more accurately characterised as a predominantly disgust-based aversion to clusters of roughly circular objects.