A fear of spiders lies deep in the human psyche, researchers say — possibly as deep as our DNA, suggesting it's something we're born with, not a learned attitude.
Their findings suggest spiders represented such a powerful threat to the survival of our distant ancestors millions of years ago in Africa that the ability to see and then avoid the eight-legged creatures became an evolutionary compulsion.
Humans who developed this DNA-level arachnophobia may have been better at survival than those who didn't learn to quickly identify the danger, scientists suspect.
"A number of spider species with potent, vertebrate-specific venoms populated Africa long before hominoids ... and have co-existed there for tens of millions of years," says Joshua New of Columbia University in New York.
"Humans were at perennial, unpredictable and significant risk of encountering highly venomous spiders in their ancestral environments," he says.
Even if a bite was not fatal, he points out, it could incapacitate someone for day or even weeks, exposing them to a number of other dangers.
In a study, researchers tested how quickly people could pick out a picture of a spider when simultaneously presented with a large number of miscellaneous images.
Most of the 250 or so people in the study could recognize spiders much more quickly than they could pick out other images people often find fearful, such as flies or hypodermic needles, the researchers found.
Because many species of venomous spiders are reclusive and hard to spot, like black widows, the ability to quickly react to their presence would be a valuable survival tool, says New.
"Detection, therefore, is the critical arbiter of success in such encounters — any improvements to the sensitivity, vigilance, reliability and speed of faculties for their detection would have been of significant selective advantage," he says.
Not everyone agrees there's an evolutionary component to arachnophobia, a deep-rooted and seemingly irrational fear of even harmless household spiders.
Jon May, a psychology professor at Plymouth University in Britain, says arachnids' long, spiky legs, dark coloring and erratic, unpredictable movements generate a feeling of unease in humans.
"We like bright-colored butterflies and ladybirds, but spiders are dark-colored with long, angular legs — and the shape and color both have strong negative associations," he says.
"Spiders just tick all these boxes, and like any phobia, when it builds up in someone's mind they can become scared even seeing a picture," he explains.