3:00 – Darwin describes his children’s first smiles in his 1872 book, The Expression of the Emotions in Man and Animals.
4:10 – On so-called Duchenne smiles, see this classic investigation.
5:00 – For a summary of “basic emotions” theory, see any number of Paul Ekman’s writing (e.g., here, here). For a recent articulation of the “social tools” theory, see writings by Alan J. Fridlund (e.g., here). For another influential recent critique of “basic emotions” theory, see here.
6:00 – For the classic bowling study, see here.
7:00 – For a recent review of facial expressions in blind people, see here.
7:45 – For a review of smiling and gender (and the importance of “rules and roles”), see here. For one of the studies linking smiliness to historical migration patterns, see here.
8:30 – For the historical shift in smiling—and its possible relation to the Kodak company—see here. For the yearbook photo analysis, see here.
9:30 – See Darwin’s discussion of infant laughter in The Expression of the Emotions in Man and Animals.
10:30 – For Darwin’s observations of laughter and smiles in primates, again, The Expression of the Emotions in Man and Animals. See Jan van Hooff’s classic study here.
11:30 – For the study comparing laughter across the Great Apes, see here. For the study of an “ape-like” stage in human laughter, see here.
12:30 – For a review of play vocalizations and laughter across species, see here.
13:20 – For the Marina Davila-Ross’s suggestion that laughter and smiles share a common evolutionary source, see here.
13:30 – For research on human infants’ open-mouthed smiles, see here.
14:00 – For the idea of the “acoustic origin” of the smile, see here.