If you suffer from an irrational fear of spiders, you may perceive the critters to be much larger than they actually are, according to a new study published in the Journal of Anxiety Disorders.

Researchers from Ohio State University recruited 57 people who suffered from arachnophobia, a fear of spiders, to better understand how perception affects phobia.  In the study, participants agreed to encounter tarantulas that varied in size (1 to 6 inches wide) five different times within an eight-week period.

In the first experiment, participants stood 12 feet away from a tank containing a spider, and moved closer to it upon instruction. Participants rated their own fear level using a distress scale of 0 to 100 as they moved closer, and once beside the tank,  researchers told them to move the spiders around with an eight-inch probe.

Afterwards, researchers took the spiders out of the room and participants were instructed to draw a single line to show how long the spider was that they saw.  Researchers found that, the more fear the participant expressed while encountering the spiders, the larger, and more inaccurate, they guessed the spiders to be.

"Given that our informal observations suggested the occurrence of the bias, we were not surprised that we found evidence for it in our study," said Michael Vasey, lead author of the study and a psychologist at Ohio State University Medical Center. "However, it is fair to say that we were very surprised by the magnitude of the bias. We have seen highly fearful participants draw lines that are two to three times as long as the actual spider."

Even in other research, Vasey said participants have looked directly at the spider while drawing the line and still estimate a larger-than-actual size.

The findings suggest that such biased perceptions may be a useful target for treatment, which could help patients recognize their observations, and then discount them and adjust for them, experts said.

Vasey said treatments for phobias are remarkably effective, although many who live in fear may not even know about them. The treatments typically come in the form of cognitive-behavior therapy, which assists the person in encountering the thing they fear so that they can correct the mistaken beliefs about the object that feeds their phobia. Nevertheless, most people who suffer from arachnophobia do not seek treatment.

"Individuals with phobias typically avoid the thing they fear or engage in safety behaviors, [or] behaviors designed to minimize risk despite encountering the feared object or situation, and therefore they are sheltered from discovering that their expectations regarding the feared object are wrong in ways that feed the fear," said Vasey.