Why do some people fear spiders?

Psychologists have said that humans tend to find unpredictable things scary. Because a spider's next move is unpredictable, it might become scary to some people. Also the fact that it jumps adds a lot more to its unpredictability. 

Psychology professor Jon May said that we tend to prefer curved features over angled ones. Because the legs of spiders are angular, they tend to look more unfamiliar and so they can be more scary to some people.

Psychologists have said that a dark colored insect is more likely to be feared than a lighter one. This might be one reason a person can find some butterflies attractive but a spider scary.

Spiders, and other insects, usually show up when they are least expected. The fact that they make an uninvited appearance can be one of the triggers of the phobia. (See Why unexpected things attract our attention?)

Children can learn to fear spiders if they see their parents responding anxiously to the appearance of one of them. This is why in some countries some people eat spiders, while in other places the same act is considered disgusting.

A child who spends more time away from nature playing with his gadgets might become more prone to the fear of spiders because they will become more unfamiliar to him in such a case.

People who love to be in control might fear insects, and specifically spiders, because they take control away from them by their unpredictability. In such a case, the fear of the spider is just a manifestation of the fear of loss of control over one's environment. (See What are the body language signs of fear?)

In a study done by Colombia University, it was found that people identify pictures of spiders faster than most other objects in the study. The study claimed that fear of spiders could be in our DNA and rooted back to the time where survival of humans depended on spotting poisonous spiders.

Psychologists say that the fear of some insects, including spiders, is connected to our disgust response. A person who gets disgusted easily might be more prone to fear spiders.

If a person was previously bitten by a spider, they might develop an anxiety towards spiders, which could later turn into a phobia. Any other traumatic experience, such as a sudden and unexpected shock, caused by a spider can result in a phobia.

Unlike many insects which usually move on the ground and can be seen moving on the wall, spiders can suddenly and unexpectedly descend from the ceiling. This additional factor adds to the fear of spiders and makes it more intense than the fear of other insects for some people.

Our brains are hard wired to detect dangers that appear all of a sudden. Because spiders usually appear unexpectedly and all of a sudden, our anxiety response is triggered and our brains might mark that as a threat.