Fear Of Spiders May Have Its Evolutionary Roots In Aversion To Scorpions

"But they won't hurt you! They eat bugs. They're our friends!" I've tried telling my now-12-year-old all these things many times over the years, but his fear of spiders persists.

It's hardly a rare fear; an estimated 6% of the general population suffer from full-blown arachnophobia. The leading explanation is that our ancestors evolved to fear spiders, and this has been passed on to us. But there are a few problems with this, point out the authors of a new paper in Scientific Reports.

Firstly: only 0.5% of spider species are potentially dangerous to humans. Secondly: these species are mostly found not in Africa, where modern humans evolved, but rather Australia and South America, the two continents that have been most recently colonised by people. Given this, the idea of an evolved, generalised fear of spiders doesn't seem that compelling.

Daniel Frynta at Charles University, Prague, and colleagues had another idea: perhaps we evolved to fear not "essentially harmless" spiders but a dangerous close relative with a similar body body plan: scorpions — and our brains over-generalise, reacting to spiders in the same way. To investigate this, they ran a study that would have left my 12-year-old with nightmares for months…

The team collected together live specimens of 62 arthropod species: 15 spiders, 10 scorpions, 5 other arachnids (such as whip "spiders"), 10 cockroaches, 10 other "hemimetabolous insects" (a group that includes earwigs, stick insects and locusts), 6 myriapods (centipedes, millipedes and their relatives), 4 beetles and 2 crabs. Each of these animals was placed in a transparent box. These boxes were then lined up for inspection by 329 participants, who used 7-point scales to rate each on scales for fear, disgust and also beauty.

Let's get beauty out of the way first: species that got high disgust ratings tended to get low beauty ratings, but there was no link between beauty and fear ratings. In other words, we can find an arthropod both scary and beautiful.

For fear, the team noted that the taxa fell into distinct groups. The "chelicerates" (the spiders, scorpions and other arachnids) got relatively high average scores, followed by the myriapods (which were definitely less fear-inducing) and finally the insects and crustaceans.

For disgust, the pattern was similar to that for fear. Myriapods did get the highest average score, but it wasn't far off the score for spiders. Again, the beetles and crabs got the lowest scores. Overall, the fear and disgust ratings were highly correlated.

Bigger species did tend to trigger stronger emotional reactions — they got higher fear, disgust and beauty scores. But the deepest statistical split in the fear-and-disgust ratings was between the chelicerates cluster (spiders/scorpions/other arachnids) and a cluster of insects-plus-crustaceans. "This supports the notion that chelicerates are perceived as one cohesive group distinct from other arthropods," the team argues.

Unlike the vast majority of spiders, scorpions do post a genuine threat, killing an estimated 2,600 people every year. Scorpions are also an ancient group, and species with a venom tailored to mammals are native to Africa and the Middle East — so our distant ancestors and dangerous scorpions could have evolved side-by-side. "Fear of scorpions therefore seems to be better warranted than fear of spiders," the team writes.

Of course, the data don't show that people generalise an evolved fear/disgust of scorpions to spiders. As the team acknowledges, this is a big limitation of the study. But it's a sensible way to interpret the data, they argue. And, compared with other theories about why so many people are so scared of spiders (over dogs, say), it does indeed seem more compelling.

Further reading

– Emotions triggered by live arthropods shed light on spider phobia