Libby Copeland writes in Slate on a study suggesting that voters aren't just looking for good-looking candidates, they favor a particular kind of good looks suggesting competence:
What does competence look like? Working with subjects rating photos of hundreds of faces, Todorov and colleagues have developed computer models of how faces can suggest character traits like trustworthiness and likability. The competent face shape is masculine but approachable, with a square jaw, high cheekbones, and large eyes. When people say Romney just looks presidential, this is the image they’re summoning.
Todorov and other psychologists believe that otherwise expressionless faces can appear to show emotion based on how they’re formed—the shape of the eyebrows can suggest anger, for instance, while a long distance between the eyes and the mouth can suggest sadness. On Todorov’s computer model of an incompetent face, beady, close-together eyes paired with high eyebrows suggest fearfulness, even through the face is expressionless. Todorov believes our tendency to read expression into neutral faces amounts to an “overgeneralization” of a healthy trait—human beings’ ability to judge others’ intentions from a brief glance.
I added the emphasis on the long distance between eyes and mouth connoting sadness because that's a standard in Byzantine iconography of Jesus Christ going back, oh, 1500 years. Spanish movie star Javier Bardem has a bit of that look to his face.