On a first pass, then, “ugliness would be the deviation from these qualities,” said David Perrett, an evolutionary psychologist at the University of St. Andrews in Scotland. Tiny, close-set eyes, prominent snout, no forehead to speak of: it sure sounds like a pig.

A helpless baby grows into a healthy, fertile youth, which in humans is visually characterized by clarity of shape, sleekness of form and visibility of musculature, said Wendy Steiner of the University of Pennsylvania, who is author of “Venus in Exile” and “The Real, Real Thing,” to be published this fall. “An animal with saggy skin, whiskers and no neck will look like some old guy who’s lost it,” she joked.

The more readily we can analogize between a particular animal body part and our own, the more likely we are to cry ugly. “We may not find an elephant’s trunk ugly because it’s so remote,” Dr. Dutton said. “But the proboscis on a proboscis monkey is close enough to our own that we apply human standards to it.” You can keep your rhinoplasty, though: the male monkey’s bulbous proboscis lends his mating vocalizations resonant oomph.

People are also keenly, even obsessively vigilant for signs of ill health in others. “That means anything that looks seriously asymmetrical when it should be symmetrical, that looks rough and irregular when it should be smooth, that looks like there might be parasites on the skin or worms under the skin, jaundice or pallor,” Dr. Miller said. “Anything mottled is considered unattractive. Patchy hair is considered unattractive.” We distinguish between the signs of an acquired illness and those of an innate abnormality. Splotches, bumps and greasy verdigris skin mean “possibly infectious illness,” while asymmetry and exaggerated, stunted or incomplete features hint of a congenital problem.

If we can’t help staring, well, life is nasty and brutish, but maybe a good gander at the troubles of others will keep it from being too short. “Deformities provide a lot of information about what can go wrong, and by contrast what good function is,” Dr. Miller said. “This is not just about physical deformities. People who seem crazy are also highly attention-grabbing.”

And as long as we’ve been gawking and rubbernecking, we’ve felt guilty about the urge. In his book “The Art Instinct: Beauty, Pleasure, and Human Evolution,” Dr. Dutton recounts a passage from Plato in which a man passing by a pile of corpses at the feet of an executioner wants desperately to look, tries to resist and then finally relents, scolding his “evil” eyes to “Take your fill of the beautiful sight!”