by Charles Darwin
Introduction, Afterword and
Commentaries by Paul Ekman

During laughter the mouth is opened more or less widely, with the corners drawn much backwards, as well as a little upwards; and the upper lip is somewhat raised. The drawing back of the corners is best seen in moderate laughter, and especially in a broad smile - the latter epithet showing how the mouth is widened. In the accompanying Figs 1-3, Plate III, different degrees of moderate laughter and smiling have been photographed. The figure of the little girl, with the hat, is by Dr. Wallich, and the expression was a genuine one; the other two are by Mr. Rejlander. Dr. Duchenne repeatedly insists that, under the emotion of joy, the mouth is acted on exclusively by the great zygomatic muscles, which serve to draw the corners backwards and upwards; but judging from the manner in which the upper teeth are always exposed during laughter and broad smiling, as well as from my own sensations, I cannot doubt that some of the muscles running to the upper lip are likewise brought into moderate action. The upper and lower orbicular muscles of the eyes are at the same time more or less contracted; and there is an intimate connection, as explained in the chapter on weeping, between the orbiculars, especially the lower ones, and some of the muscles running to the upper lip. Henle remarks, on this head, that when a man closely shuts one eye he cannot avoid retracting the upper lip on the same side; conversely, if any one will place his finger on his lower eyelid, and then uncover his upper incisors as much as possible, he will feel, as his upper lip is drawn strongly upwards, that the muscles of the lower eyelid contract. In Henle's drawing, given in woodcut, Fig. 2, the muscularis malatis (H) which runs to the upper lip may be seen to form an almost integral part of the lower orbicular muscle.

I disagree with Darwin on the anatomical basis for the facial movements he proposes in this paragraph. I believe Duchenne was right and Darwin was wrong about why the upper lip is often raised in a strong smile. My research suggests that with enjoyment the zygomatic major muscle is the principal muscle in the lower face, and may be the only active muscle in the lower face. The exposure of the teeth and raising of the upper lip noted by Darwin can occur by strong activation of zygomatic major alone, and does not, as Darwin proposed, require the activation of a second muscle. The strong action of the zygomatic muscle stretches the upper lip, and in most people this also slightly raises the lip. Of course, additional muscles can he activated which will raise the upper lip further, but this is not typical or necessary in a smile of enjoyment. I believe Darwin was also mistaken in accepting Henle's claim that eye-closures raise the upper lip. The upper lip is often slightly raised, particularly at the outer edges, but it is the action orbicularis oculi alone which stretches the skin and causes this change in the upper lip, not the activation of another muscle in the lower face.

The man who appears in Figures 4, 5 and 6 (below) also appears in later illustrations. Duchenne wrote about him, "The individual I chose as my principal subject for the experiments . . . was an old toothless man, with a thin face, whose features without being absolutely ugly, approached triviality and whose facial expression was in perfect agreement with his inoffensive character and his restricted intelligence. . . . There are few people who are willing to submit to this type of experiment, because, without being extremely painful, electrilization of the facial muscles often provokes involuntary movements resulting in contortion of the facial features. This subject had reduced sensations. He was suffering from a complicated anesthetic [unable to sense pain] condition of the face. I was able to experiment on his face without causing him much pain, to the extent that I could stimulate his individual muscles with as much precision and accuracy as if I were working with a still irritable cadaver."

Dr. Duchenne has given a large photograph of an old man (reduced on Plate III, Fig. 4), in his usual passive condition, and another of the same man (Fig. 5), naturally smiling. The latter was instantly recognized by every one to whom it was shown as true to nature He has also given, as an example of an unnatural or false smile, another photograph (Fig. 6) of the same old man, with the corners of his mouth strongly retracted by the galvanization of the great zygomatic muscles. That the expression is not natural is clear, for I showed this photograph to twenty-four persons, of whom three could not in the least tell what was meant, whilst the others, though they perceived that the expression was of the nature of a smile, answered in such words as "a wicked joke," "trying to laugh," "grinning laughter," "half-amazed laughter," etc. Dr. Duchenne attributes the falseness of the expression altogether to the orbicular muscles of the lower eyelids not being sufficiently contracted; for he justly lays great stress on their contraction in the expression of joy. No doubt there is much truth in this view, but not, as it appears to me, the whole truth. The contraction of the lower orbiculars is always accompanied, as we have seen, by the drawing up of the upper lip. Had the upper lip, in Fig. 6, been thus acted on to a slight extent, its curvature would have been less rigid, the naso-labial furrow would have been slightly different, and the whole expression would, as I believe, have been more natural, independently of the more conspicuous effect from the stronger contraction of the lower eyelids. The corrugator muscle, moreover, in Fig. 6, is too much contracted, causing a frown; and this muscle never acts under the influence of joy except during strongly pronounced or violent laughter.