We propose as musical universal an imbalance in the the system of differences that exist within a piece of music. Differences in music are characterized by the presence of features occurring at some place in the music, but not elsewhere. One class of features are those musical configurations or shapes that are descriptively as simple as can be (e.g. isochronic (pulsed) rhythm). Such shapes can recur in (temporally) longer or shorter forms: the longest such form is that shape's "maximal" occurrence. Our evidence suggests that for music in general: 1) maximals are never uniformly distributed, 2) there is a maximal corresponding to the greatest increase in motion, and 3) this tends to occur after the halfway mark of the piece: a "motion asymmetry." This is consistent with Huron's finding that repetition occurs most frequently near the beginning of a piece.
Motion asymmetry is present in the schema: motif, motif, get faster, stop. In a high percentage of tunes in the ESAC collection, this schema is present in the whole or a part of the piece (very typically as an opening phrase); many classical "sentences" and indeed whole works also exhibit this shape.
The question is: what accounts for its frequency? Possibly music cognition depends on a neural substrate of "motion detection" . We argue that asymmetric motion is a marker of animate agency. This implies that an indispensable role of form is to produce the relevant markers.