Why are so many people afraid of spiders?

Spin in wastafel

Harmless 

Spiders - those indigenous to the Netherlands, at least - are harmless, so that is not the problem. However, many insects, spiders and snakes were dangerous to our hunter-gatherer ancestors.

Spider-fearing ancestors

A valid argumentation is that our ancestors were less likely to kick the bucket prematurely if they quickly figured out that spiders (or snakes) could be dangerous and avoided them out of fear. This gave them a bigger 'reproductive success': they were more likely to produce offspring. However, the offspring of these spider-fearing hunter gatherers would also inherit the tendency to be quickly afraid of spiders. This hereditary effect would have to be following us for over 100,000 years. Our DNA does not differ much from the hunter-gatherer homo sapiens, which explains why we have this fear of spiders today.

Scary stories

Psychologists have examined this in humans. For even though it is easier to teach humans to fear spiders than to fear flowers, that might still just be because people heard many scary stories about spiders. The experiments on monkeys are the most convincing: it is way easier to teach them to fear (fake) snakes than to fear (fake) flowers.

Fear of spiders appears to be an evolutionary hangover.