Why can't we walk straight? A group led by Jan Soulman of the Max Planck Institute for Biological Cybergenetics in Germany recently made gains toward answering this age-old question. By conducting a series of experiments with blindfolded test subjects, the team systematically ruled out body asymmetries as an explanation: Things like uneven legs and right- or left-side dominance did not correlate with walkers' veering directions. They also ruled out random noise in sensory input and/or motor output as the culprit, since this would have caused walkers to meander back and forth in a zig-zag fashion rather than to trace out circles.
Loopy paths, they concluded, are caused by a walker's changing sense of "straight ahead" itself. With every step, a small deviation is added to her cognitive sense of what's straight, and these deviations accumulate to send her veering around in tighter and tighter circles as time goes on. This increasing curvature doesn't happen under normal circumstances, which is to say, when external reference points are visible, because those allow the walker to frequently recalibrate her sense of direction.
Soulman's team is not quite sure where in our inner workings the accumulating deviations arise. Their best bet is placed on the vestibular system -- the system in our inner ears that maintains balance -- which is already well-known to exhibit biases. Some people have vestibular disorders so severe that they find walking in straight lines impossible even under normal circumstances. For most of us, the subtle leftward or rightward bias of our sense of direction would only rear its head if we were trying to find our way through a dense forest, or, perhaps, blindfolded by pirates and made to walk the plank.