" Though we may show a polite grin or camera smile at will, the zygomatic or heartfelt smile is hard to produce on demand. While the former cue may be consciously manipulated (and is subject to deception), the latter is controlled by emotion. Thus, the zygomatic smile is a more accurate reflection of mood. "

― David B. Givens , The NONVERBAL DICTIONARY of gestures, signs and body language cues