Malcolm Gladwell’s Blink cites Paul Ekman’s work on the Facial Action Coding System (FACS):
Facial Action Coding System (FACS) is the most widely used and versatile method for measuring and describing facial behaviors. Paul Ekman and W.V. Friesen developed the original FACS in the 1970s by determining how the contraction of each facial muscle (singly and in combination with other muscles) changes the appearance of the face. They examined videotapes of facial behavior to identify the specific changes that occurred with muscular contractions and how best to differentiate one from another. They associated the appearance changes with the action of muscles that produced them by studying anatomy, reproducing the appearances, and palpating their faces. Their goal was to create a reliable means for skilled human scorers to determine the category or categories in which to fit each facial behavior. The FACS Manual was first published in a loose-leaf version with video or film supplements in 1978.
A sample page from the manual describes Action Unit 1, the Inner Brow Raiser:
One large muscle in the scalp and forehead raises the eyebrows. It runs vertically from the top of the head to the eyebrows and covers virtually the entire forehead. The medial (or central) portion of this muscle (AU 1) can act separately from the lateral portion of this muscle (AU 2). Figure 2-1 shows that the movement of AU 1 is to pull the medial part of the brow and center of the forehead upwards.