Two American studies suggest that some people start shaking atop skyscrapers, or feel their knees buckle going over bridges, because they significantly overestimate vertical distances.
The researchers behind the study say that the stronger the fear, the bigger the error would be.
Russell Jackson, a cognitive psychologist at California State University in San Marcos who led one of the studies, says that the new finding runs counter to traditional theories of acrophobia.
"Acrophobia is an excessive fear in response to something that's perceived normally," New Scientist magazine quoted him as saying.
However, he added while talking about his new results: "An important component of acrophobia appears to be that they are perceiving something different in the first place (and reacting normally)."
Jackson's study involved 43 students, who had previously filled out a psychological survey that included questions to gauge acrophobia.
During the survey, the subjects were asked to rate their anxiety over situations like crossing a bridge or riding a Ferris wheel.
With an eye on testing perception, Jackson and his colleagues asked each volunteer to approximate the height of a five-storey, 14.4-metre parking garage.
The subjects were placed either at the top or bottom of the building, and a research assistant marched away slowly.
Jackson's team would take note when the subjects felt that the assistant had paced a distance equal to the height of the building.
The researchers found that all but one volunteer overestimated height, whether from the top or bottom of the building.
The subjects, however, seemed better while judging the distance from the bottom of the building than the top, with those most scared of heights judged the building 3 metres higher from the bottom, and 12 metres higher from the top, as compared to those who scored lowest on the acrophobia test.
Because subjects erred both on top of the building and while safe on the ground, Jackson says that their fear seems to be driven primarily by misperception.
The researcher says that acrophobiacs who see a 14-metre building like it's 50 metres react like normal people would to a 50-metre building.
"There's no-one that's fearless when it comes to heights," he says.
Jeanine Stefanucci, a psychologist at the College of William and Mary in Williamsburg, Virginia, led the second study.
She also agrees that misperception is key to acrophobia, but thinks that fear drives misperception and not the other way around.
She and her colleagues asked the subjects to judge vertical distance above a two-storey balcony.
The researchers observed that upon being shown provocative images like guns or snakes, the subjects tended to misjudge vertical, but not horizontal distances.
However, the subjects started to judge vertical distances more accurately after suppressing their gut reactions to the images, while their misjudgements increased when they reacted strongly to the photos.
Stefanucci says that that observation indicated that fear was driving misperception, but admitted that the relationship could more complex than one causing the other.
It is believed that, either way, both team's findings offer a new approach to treating acrophobia.
While Jackson's study has been published in the Proceedings of the Royal Society B, Stefanucci's study has been reported in the Journal of Experimental Psychology.