As evil as it looks, science says Jared Loughner's creepy grin could very well be the mark of a scared kid.
Despite what those "Don't Worry, Be Happy" T-shirts may tell you, the smile is actually a very complex expression. It's used to convey cheer, of course, but remember that smiling can also be used as a defense mechanism in times of distress.
If you'd prefer to ignore the mountains of anecdotal evidence about nervous smiling in tense situations, consider this from Anthony Stocks, chairman of the anthropology department at Idaho State University:
The evolution of smiles is opaque and, as with many evolutionary accounts of social behavior, fraught with just-soism. Among human babies, however, the 'tooth-baring' smile is associated less with friendship than with fright--which, one might argue, is related to the tooth-baring threats of baboons. ... Somehow we seem to have taken the fright-threat sort of smile and extended it to strangers as a presumably friendly smile. Maybe it is not as innocent as it seems.
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Other science says that people in the presence of authority figures—say an accused killer surrounded by police—frequently laugh unconsciously at unfunny things in an effort to get into the authority figure's good graces.
None of this is to say that Loughner is definitely not a homicidal maniac laughing at his carnage, of course. But I think what's more likely is that he's just a scared kid who's seeing the end of his whole world in that camera lens.