Come closer. Standing 1 meter away from a virtual reality drop-off (top) makes a slope appear steeper than when one stands at the edge (bottom).

Zhi Li

Don't Fear the Edge

Some advice for those scared of heights: stand closer to the edge. A new study reveals that downhill slopes appear less steep the closer you get to the drop-off.

Psychologist Frank Durgin and his colleague Zhi Li first made the observation while walking around their campus at Swarthmore College in Pennsylvania. "There's a creek that has very high banks that go down very steeply on both sides," says Durgin, who studies how humans perceive visual information in natural environments. "So we went out to one of these paths along the top of one of these banks and just tried to see what happens [as we moved away]." The duo found that the banks appeared steeper as they retreated from the edge.

Durgin and Li took their puzzling observation back to the lab. They asked dozens of students to estimate the angle of a slope as they stood at various distances from a piece of inclined plywood. The students lifted up a smaller plywood board, hooked to a fishing line, to reflect what they thought the angle of the slope was. Then, in a virtual environment, the volunteers guessed the angle of a slope as they stood on top of what looked like a 14.5-meter-tall hill that plunged into a large lake. In both cases, as participants inched closer to the drop-off, they more accurately reported the slopes as less steep--ultimately by about 8 degrees, the team reported last month in the Journal of Vision.

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Watch out! See a virtual reality demonstration of the slope illusion.
Zhi Li

Durgin chalks the effect up to how we move our eyes and head. A geometric model he created shows that as we approach hills our head and gaze tilt downward, making the slope seem shallower than our initial gazes suggested.

The results are surprising, says Bruce Bridgeman, a psychologist at the University of California, Santa Cruz, because they contradict the "danger hypothesis." To keep us safe, he explains, one would expect our eyes to exaggerate the danger of a steep slope the closer we got to it. "It's actually the other way around."