The difference between facial expressions of genuine and make believe emotions was first noted by Charles Darwin in e Expression of the Emotions in Man and Animals, published in 1872. 12 Darwin was aware of observations made a decade earlier by Guillaume-Benjamin Duchenne about the musculature involved in smiling and the type of control needed to move that musculature.
Duchenne determined that a smile of real joy required the combined involuntary contraction of two muscles, the zygomatic major and the orbicularis oculi. (See g. 7-4') He discovered further that the latter muscle could be moved only involuntarily; there was no way of activating it willfully. The involuntary activators of the orbicularis oculi, as Duchenne put it, were "the sweet emotions of the sou!." As for the zygomatic major, it can be activated both involuntarily and by our will and is thus the proper avenue for smiles of politeness.
Descartes´ Error. Antonio Damasio.