The eyes' romantic depiction as the "windows to the soul" isn't just the stuff of whimsical verse. Sure, the word pupil comes from the Latin for "little doll," referencing how those storied orbs produce miniature, doll-like reflections of people in their sightline, much like shiny sunglasses lenses [source: Merriam-Webster]. But the vacillating openings at the center of our irises indeed mirror more than what's on the outside.

The mechanism of pupil constriction and dilation is regulated by the autonomic nervous system, which is also responsible for other uncontrollable reactions like goose bumps and heart rates. Inside the eyeball, the dilator and the sphincter muscles play the iris tissue like an accordion to the tune of light [source: Swaminathan]. Then again, the sun isn't the only thing orchestrating when the pupils open and shut. Humans' inborn fight or flight response, triggered by the parasympathetic nervous system -- a subset of the autonomic nervous system -- also manifests in our irises. When fear strikes, as Charles Darwin first proposed in the 1870s, the pupils expand to heightened attention and focus [source: Stern, Ray and Quigley]. About a century later, scientists discovered that pupils also pop when we experience emotions on the sunnier end of the spectrum.

In a 1965 Scientific American magazine article, psychologist Eckhard Hess described an intriguing experiment [source: Stern, Ray and Quigley]. While showing his research assistant James Polt a series of photographs, Hess tracked changes in the diameter of Polt's pupil size. Lo and behold, Polt's pupils enlarged most dramatically when a picture of a nude woman flashed before his eyes, leading Hess to hypothesize that sexual arousal stimulates the pupils [source: Andreassi]. Further experimentation found heterosexual people's pupils dilated when staring at opposite-sex nudes, whereas homosexual participants exhibited that pupillary response when looking at same-sex nudes, offering further confirmation of a link between sexual interest and dilation [source: Andreassi].

Additionally, the researchers noticed a compelling clue about how the eyes may influence physical attraction. Not only do pupils dilate in response to titillating material, but people also rate faces with saucer-like pupils as more attractive than those looking back with fuller irises [source: Murphy].