As many artists have discovered, the smile is quite difficult to render and can come with an almost unlimited degree of nuance. Realistic smiles, in particular, which are judged in terms of sincerity and inflection, require an understanding of how to depict tiny differences in detail.
Artists who create characters with stylized faces tend to be quite conservative with their smiles, sticking very close to the human model. From the point of view of aesthetics or character design, this can make perfect sense, but it turns out that stylized smiles can be successfully expressed with an array of shapes that can be both crude and only vaguely connected to human anatomy. This is clear in my sample array of emoticons which all get the joy message across quite effectively, with the exception of the squiggly line with the two mismatched eyes, where it’s not at all clear what expression the artist intended.
Closed-mouth Smile ("U")
(Triangle or Wedge)
Clearly, there are certain graphic triggers that we require in order to see smiles, but I’m not completely clear as to all the elements involved, or how they succeed in suggesting the anatomy of the real smile. Here’s my conjecture: in these three examples there are two factors at work, the "U" which involves a curved line, inflected so that it bows upwards. This simplification most likely works because it suggests the curve of the line between the lips in the closed-mouth smile - see my realistic drawing, left.
The other version, the wedge or triangle, requires a straight upper edge, and a lower border which is narrower than the upper. It also must be angular, simulating the tight, stretched look of the upper lip of the open-mouth smile - see my realistic drawing, left.
(The wiggly line in the failed emoticon is perhaps an attempt to suggest a slack or twisted mouth, but neither of those configurations is associated with joy.)
Of course, all bets are off when we deal with human or very realistic faces, when we use an entirely different brain mode to do our analysis, one which is much pickier. But as far as stylized smiles go, artists can be, and have been extremely inventive; look at what the geniuses at Disney did to make Donald Duck, with his rigid beak, and Winnie the Pooh with his big curved line for a smile, appear happy.
More about Donald and the fascinating world of smiles in my next several blog posts.