Left-sided oral asymmetries in spontaneous but not posed smiles.

To investigate possible facial asymmetries during the production of posed and spontaneous smiles, the displacement of various reference points on the mouth were measured as subjects produced both kinds of smiles. Strobe cameras were used in combination with a computer-based analysis to record the smiles of left- and right-handed males and females. The analysis revealed that the left side of the mouth moved more than the right side during spontaneous but not posed smiles, supporting the notion that the right hemisphere may play a special role in emotional expression. This asymmetry was most apparent in left-handed females and right-handed males. These sex and handedness differences are discussed with reference to apparent inconsistencies in previous research on asymmetries in emotional expression.