Face symmetry

Face symmetry is how closely the left and right halves of a face correspond across its vertical midline. It is measured by pairing features that mirror each other — the eye corners, brows, cheekbones, jaw corners, nostrils and mouth corners — and totalling how far their positions differ. Nobody is symmetric: every published measurement finds a typical deviation well above zero in every population, and small asymmetries are universal rather than notable. Symmetry does correlate with rated attractiveness in the literature, but weakly, and the effect shrinks as photographs get better — which suggests part of it was always measurement noise. The largest practical source of error is head rotation: turning a few degrees compresses one side of the face in the image, and a symmetry tool measures the image. Comparable readings need the same pose and the same light.