When people feel disgust or sadness, their pupils involuntarily widen. Conversely, feelings of anger are associated with a narrowing of the pupils, setting it apart from other negative mental states. These distinct physical responses occur even when individuals experience mixed feelings at the exact same moment. The research was published in Biological Psychology.
The size of the human pupil is controlled by the autonomic nervous system, which governs involuntary body functions. Pupil dilation reflects the activation of the sympathetic nervous system. This is the same biological network responsible for the body’s fight or flight response. When this system springs into action, it engages a specific muscle in the iris that pulls the pupil open. A wider pupil increases a person’s overall field of vision, allowing them to scan the broader environment for potential threats.
Pupil constriction relies on the parasympathetic nervous system, which usually helps the body rest and digest. When this system takes over, the dilating muscle relaxes and another muscle pinches the pupil shut. A smaller pupil narrows the field of view. This creates a sharper image, giving the eye better visual acuity for fine tasks or focused attention.
Historically, researchers have debated how human emotions map onto these physical changes. One school of thought suggests that emotions are just broad, culturally learned labels for general states of high or low arousal. In this view, a person simply feels a general sense of negativity, which they might label as anger or disgust depending on the situation. Another school of thought argues that basic emotions are biologically distinct categories. Under this specific framework, anger, fear, and sadness should each possess a unique physiological signature.
Most prior research measuring pupil size looked only at broad emotional categories. Studies would expose people to positive or negative pictures and measure the resulting eye movements. These tests reliably showed that negative images correspond with wider pupils. Yet the previous tests did not ask the participants exactly what they were feeling at any given moment.
University of Suffolk psychologist Kate McCulloch and her colleagues suspected that relying on broad stimulus categories might obscure the finer details of emotional biology. They designed an experiment focused entirely on personal, distinct emotional experiences. Instead of assuming an image was universally scary or sad, the team asked individuals to rate their own internal states.
For their first experiment, the researchers recruited 98 participants. The volunteers viewed 36 images and listened to 18 short audio clips. The researchers selected these materials from standardized scientific databases containing emotionally engaging content. The visual materials ranged from neutral pictures of filing cabinets to intense images of aggressive animals or severe injuries.
Testing visual reactions requires strict controls because light naturally changes pupil size. To account for this, the researchers altered the images so they all shared the exact same average brightness and contrast. They also generated scrambled, completely unrecognizable versions of every picture. This allowed the team to verify that any physical eye reactions were tied to the emotional content of the picture, not just its basic color palette.
During the testing phases, participants kept their heads completely still using a chin support. Specialized infrared cameras tracked their eye movements and recorded the exact area of their pupils. After viewing a picture or hearing a sound, the participants used a keypad to rate their immediate feelings on a scale from one to nine. They scored how strongly they felt five specific emotions: happiness, sadness, anger, fear, and disgust.
This rating system accommodated the reality of mixed emotions. A participant might view an image of an injured animal and feel a combination of deep sadness and mild disgust at the same time. By recording multiple ratings for a single event, the researchers could use statistical models to isolate the specific influence of each separate feeling.
McCulloch and her team found that personal ratings of disgust and sadness reliably predicted larger pupil sizes. When participants reported feeling higher levels of disgust than their own general baseline, their pupils dilated. The pupil widening effect for disgust began roughly two seconds after the start of a prompt and lasted for the entire viewing or listening period.
Fear was associated with pupil widening only when participants listened to audio clips, and this reaction happened very late in the recordings. The pupil’s response to anger did not reveal any obvious pattern during this first round of testing. The statistical links between anger and pupil size were not statistically significant for the images or the sounds in the initial experiment.
To see if these specific emotional patterns would hold up under different conditions, the researchers conducted a second experiment with 102 new participants. They discarded the visual images entirely. By only using audio clips, the team completely eliminated any lingering concerns about screen brightness interfering with the eye cameras.
The team also swapped the short, standardized sounds for 30-second clips taken from movies, television shows, and online videos. Some clips featured soothing nature sounds, while others contained aggressive arguments, crying out, or visceral noises like vomiting. Participants listened strictly to the audio while staring at a simple cross on a blank computer monitor. Just as before, they rated their five emotional states immediately afterward.
The second experiment confirmed the most prominent patterns from the first. High personal ratings of disgust again emerged as the strongest predictor of pupil dilation. The sustained physical reaction to feeling disgusted lasted throughout the entire half-minute audio clip. Sadness also predicted pupil widening, though to a slightly lesser extent than disgust.
The longer audio format exposed a totally different pattern for anger. When participants reported feeling angry during the extended clips, their pupils reliably shrank. This physical constriction effect completely sets anger apart from the other measured negative mental states. Happiness was associated with a slight widening of the eye during these tests, though the association was milder than the one connected to disgust.
Disgust and anger are often difficult to separate in psychological experiments. Some researchers have argued that the two emotions are practically interchangeable responses to things we dislike. Because anger predicts the narrowing of the pupil while disgust predicts its widening, they appear to trigger two entirely different nervous system responses.
These distinct physiological signatures align with evolutionary theories of behavior. Anger typically readies the body to confront a specific target. This sort of focused approach requires narrow, precise vision. Disgust usually prompts the body to back away or scan the wider environment to avoid a disease or contaminant, which requires a broader visual field.
The authors noted a few limitations regarding their methodology. All data collection occurred in a controlled laboratory setting. Artificial audio clips and images may not provoke the exact same physical intensity as real threats encountered in daily life. The study design also relied entirely on self-reporting. This method depends heavily on a person’s ability to accurately identify and describe their own internal moods.
Future experiments might track eye changes after the sights and sounds completely end. Some emotional reactions involving fear appeared exclusively at the very end of the longer audio clips. Tracking these delayed, post-exposure responses might reveal new details about how the human body processes different types of psychological distress.
The study, “Differences in Pupil Size During Self-Reported Experiences of Disgust, Sadness, Fear, Anger, and Happiness,” was authored by Kate McCulloch, Edwin S. Dalmaijer, Gerulf Rieger, and Rick O’Gorman.
