Why do our eyes open wide when we feel fear or narrow to slits when we express disgust? According to new research, it has to do with survival.

In a paper published in the journal Psychological Science, researchers concluded that expressions of fear and disgust altered the way human eyes gather and focus light. They argued these changes were the result of evolutionary development and were intended to help humans survive, or at least detect, very different threats.

To test their hypothesis, study authors examined two dozen volunteer undergraduate students with standard eye-exam equipment, and asked them to mimic expressions of fear and revulsion.

Researchers found that when the students widened their eyes, their corneas admitted more light and expanded their field of vision. When they wrinkled their noses in disgust, their eyes narrowed. This had the effect of blocking out more light but sharpening focus on a specific point.

Although some scientists have proposed that emotional expressions are intended primarily to communicate information, study authors argued that expressions of fear and disgust seem to perform different visual functions.

“Eye widening may improve detection and localization of a potential threat that requires enhanced vigilance, which would be consistent with the hypothesized function of fear,” wrote senior author Adam Anderson.