People don't keep their heads still when looking to one side, and other people can tell.

Twenty pairs of photographs were made of adults looking 25 degrees to the left and 25 degrees to the right while attempting to face forwards. The eye regions of each photograph were concealed. Twenty adults attempted to sort each pair into left-looking and right-looking pictures. They were successful 65% of the time, p < 0.001. This suggests models have difficulty looking to one side without a perceptible head turn or comparable facial cue. This previously unrecognised phenomenon has implications for research on detection of gaze.