the duchenne smile
"The emotion of frank joy is expressed on the face by the combine contraction of the zygomatic major muscle and the orbicularis oculi. The first obeys the will but the second is only put in play by the szeet emotions of the soul; the... fake joy, the deceitful laugh, cannot provoke the cotraction of this latter muscle... The muscle arounde the eye does not obey the will; it is only brought into play by a true feeling, by an agreable emotion. Its inertia in smiling unmasks a false friend" Duchenne de Boulogne, 1862.