Evidence for Axis-Aligned Motion Bias: Football Axis—Trajectory Misalignment Causes Systematic Error in Projected Final Destinations of Thrown American Footballs

The axis-aligned motion (AAM) bias is the tendency of observers to assume that symmetric moving objects maintain axis–trajectory alignment and to bias their judgments of trajectory toward the axis when they are misaligned. We tested whether humans exhibit an AAM bias in a realistic, cue-rich, 3-D setting by examining the impact of axis–trajectory misalignment on estimates of final destinations of thrown American footballs. In experiments 1 and 2 we show that observers are significantly worse in judging destinations of footballs than those of volleyballs and basketballs. This difference in performance is due to the deviation of the football's axis from trajectory in flight, as shown by the correspondence of participants' lateral judgment error and the football's lateral axial deviation from trajectory, which was predicted by passer handedness. Nearly all animals exhibit bilateral symmetry and maintain axis–trajectory alignment during locomotion, and we argue that the AAM bias is complementary mental attunement to the natural regularity of this axis-aligned motion. Furthermore, this bias is also a prototypical example of a perceptual regularity that is a mixed blessing—advantageous in perceptual judgment tasks of axis–trajectory-aligned moving entities like most living creatures, and disadvantageous in tasks demanding judgments of axis–trajectory-misaligned moving objects which are typically artifacts.