Your pupils dilate when you look at someone because your autonomic nervous system responds to emotional arousal, and other people are among the most emotionally stimulating things you encounter. The dilation is not unique to romantic attraction, though that is the context most people think of. Any strong internal state, from interest to disgust to the simple mental effort of reading someone’s facial expression, can trigger the same widening. What makes person-to-person eye contact special is that it layers several of these triggers at once, and it activates a remarkable phenomenon where your pupils actually mirror the other person’s.
How the Brain Widens Your Pupils
Your pupil size is controlled by two competing branches of the nervous system. The parasympathetic branch constricts the pupil, mostly in response to bright light. The sympathetic branch dilates it, working in direct opposition to that constriction signal.1PubMed Central. Functional Organization of the Sympathetic Pathways Controlling the Pupil: Light-Inhibited and Light-Stimulated Pathways Pupils respond to three distinct kinds of input: they shrink in bright light, shrink when you focus on something up close, and grow when arousal or mental effort increases.2PubMed Central. Pupillometry: Psychology, Physiology, and Function That third category is the one that matters here.
Deep in the brainstem sits a tiny cluster of neurons called the locus coeruleus. It is the brain’s main source of norepinephrine, a chemical that ramps up alertness and attention. When the locus coeruleus fires, norepinephrine floods the brain, and one visible side effect is that your pupils get bigger. Direct recordings from this brain region show that changes in locus coeruleus activity reliably predict changes in pupil diameter, both when those changes happen spontaneously and when they are triggered by something in the environment.3Neuron. Relationships between Pupil Diameter and Neuronal Activity in the Locus Coeruleus, Colliculi, and Cingulate Cortex Researchers now treat pupil size as a real-time readout of this system’s activity, which is why pupil measurement has become a standard tool in psychology and neuroscience labs.4PubMed Central. From pupil to the brain: New insights for studying cortical plasticity through pupillometry
It Is About Arousal, Not Just Attraction
The popular version of this phenomenon focuses on romantic interest: “your pupils dilate when you see someone you’re attracted to.” That is not wrong, but it is incomplete. Pupils dilate to emotionally arousing stimuli regardless of whether the emotion is pleasant or unpleasant. Research measuring pupil responses while people viewed emotionally charged images found that both pleasant and unpleasant content produced larger pupil changes than neutral content, and that these changes tracked closely with skin conductance, a standard measure of sympathetic nervous system activation.5PubMed Central. The pupil as a measure of emotional arousal and autonomic activation Your pupils don’t know whether you’re looking at someone you love or someone who frightens you. They just respond to the intensity of the feeling.
More recent work has tried to tease apart which specific emotions drive the biggest pupil responses. Across two studies involving 200 participants who rated their own emotional reactions to images and sounds, self-reported disgust was consistently the strongest predictor of pupil dilation, followed by sadness. Happiness had a weaker effect, and fear showed up mainly as a delayed response.6PubMed. Differences in pupil size during self-reported experiences of disgust, sadness, fear, anger, and happiness So when your pupils widen while looking at someone, it could reflect desire, but it could just as easily reflect alarm, fascination, surprise, or even revulsion. The pupil is a sensitive indicator of how much processing your brain is devoting to what it sees, not a lie detector for romantic feelings.
The Hess Experiments and the Myth They Created
The idea that dilated pupils signal sexual or romantic interest traces back to the psychologist Eckhard Hess, who published a series of influential papers in the 1960s and 1970s. His early work reported that pupils dilate when people view interesting or emotionally engaging visual stimuli, and he noted differences between men and women in which kinds of images produced the biggest responses.7PubMed. Pupil size as related to interest value of visual stimuli These findings were exciting and widely cited. They also planted the seeds of a pop-psychology truism: that you can tell if someone likes you by watching their pupils.
The problem is that later replication efforts have painted a messier picture. A systematic attempt to replicate several of Hess’s key findings confirmed some effects but not others, and the overall picture suggests that Hess was right about the general principle (arousing stimuli dilate pupils) but oversimplified the link to specific emotional states like romantic interest.8PubMed. Replicating five pupillometry studies of Eckhard Hess The arousal signal is real. The idea that you can use it as a reliable cue for attraction in everyday life is a stretch, because too many other things (room lighting, cognitive effort, emotional state) influence pupil size at any given moment.
That said, there is solid evidence that pupil dilation tracks sexual arousal specifically when people view sexual content. Studies using pupillometry to measure responses to sexual imagery have found that pupil dilation serves as an indicator of arousal that corresponds with observers’ sexual orientation.9PubMed Central. Pupillary Response as an Age-Specific Measure of Sexual Interest In a lab setting with controlled stimuli, the signal is clear. In a dimly lit restaurant, it is drowned out by noise.
Pupil Contagion and Why Eye Contact Feels Intense
One of the most striking discoveries in this area is that people unconsciously match each other’s pupil size. This phenomenon, called pupillary contagion, works a bit like emotional contagion: when you see someone whose pupils are dilated, your own pupils tend to dilate too, and the effect is accompanied by physiological arousal markers like changes in heart rate.10PubMed Central. Pupil contagion variation with gaze, arousal, and autistic traits This happens without any conscious awareness. You do not decide to match the other person’s pupils. Your nervous system just does it.
The effect is strongest when you are actually looking at someone’s eye region, which helps explain why direct eye contact with another person feels qualitatively different from just glancing at them. When gaze is restricted to the eyes, the contagion effect and its physiological correlates are more pronounced. In other words, eye contact does not just let you see the other person’s emotional state through their pupils; it pulls your own body into a matching state of arousal. This may be one reason why sustained eye contact with someone you find attractive feels so charged. You are not only responding to them; you are amplifying each other’s arousal in a feedback loop.
Do Dilated Pupils Actually Make People Look More Attractive?
If dilation signals interest, do observers pick up on that signal? The evidence here is surprisingly mixed. Research from as far back as Hess’s era suggested that men in particular find women with dilated pupils more attractive, and a study examining real-world consumer behavior found that a significantly greater percentage of men than women chose a magazine cover featuring a model with larger pupils.11PubMed. Judging a book by its cover: the unconscious influence of pupil size on consumer choice The influence appeared to be unconscious: most participants could not articulate why they preferred one image over another.
However, a more recent series of experiments complicates this picture. Across multiple studies, researchers found that people actually appeared more attractive when their pupils were constricted, not dilated.12PubMed. Beauty is in the iris: Constricted pupils (enlarged irises) enhance attractiveness The proposed explanation is that smaller pupils make the colored iris more visible, and it may be the iris, not the pupil, that drives attractiveness judgments. This contradicts decades of pop-psychology advice and even some older scientific claims, and the research is still being debated. The honest answer right now is that the relationship between pupil size and perceived attractiveness is more complicated than the simple “bigger pupils = more attractive” story that gets passed around.
Belladonna and the History of Artificial Dilation
People have intuited a connection between pupil size and allure for centuries. During the Italian Renaissance, Venetian women applied eye drops made from the juice of the belladonna plant to artificially dilate their pupils, believing it made them look more seductive. The plant’s name, belladonna, literally translates to “beautiful woman,” and the most likely origin of that name is exactly this cosmetic practice. Belladonna contains atropine, a compound that blocks the parasympathetic signals that constrict the pupil, leaving the sympathetic dilating pathway unopposed. The result is dramatically widened pupils that can last for hours. Ophthalmologists still use atropine-like drops today to dilate eyes during exams, which is why the practice feels both ancient and oddly modern.
The Role of Oxytocin
Oxytocin, sometimes called the “bonding hormone,” appears to modulate how the pupil responds during social interactions. When oxytocin is administered via nasal spray, it enhances pupil dilation in response to emotionally relevant social stimuli such as faces.13PubMed Central. Oxytocin enhances pupil dilation and sensitivity to ‘hidden’ emotional expressions This suggests that oxytocin doesn’t directly cause dilation but rather turns up the dial on how much attention the brain pays to social information, which in turn drives pupil responses.
In a study on the effects of oxytocin on emotion recognition, oxytocin treatment improved people’s ability to identify emotions in faces, and this improved performance came with increased task-related pupil dilation, pointing to a greater recruitment of attentional resources.14PubMed. Effects of intranasal oxytocin on pupil dilation indicate increased salience of socioaffective stimuli This was especially pronounced for face stimuli that participants would normally not devote much processing to, such as male faces viewed by male participants. In a study on children with autism receiving chronic oxytocin administration over four weeks, the treatment induced a relative increase in pupil dilation during live social interactions, and the heightened autonomic responsiveness showed a moderate association with the children’s self-reported feelings of secure attachment.15PubMed. Impact of chronic oxytocin on gaze and pupil dynamics during live dyadic interactions in children with autism
These findings add a hormonal layer to the story. When you look at someone you feel close to, your brain is likely releasing oxytocin, which amplifies how much attentional weight you give their face, which drives greater pupil dilation. The dilation you notice during eye contact with a partner or a close friend is probably not just arousal in the abstract. It reflects the brain actively investing more processing power in the person in front of you.
When Pupil Responses Work Differently
Not everyone’s pupils respond to social stimuli in the same way, and these differences can be informative. A systematic review and meta-analysis of pupillary responses in autistic versus non-autistic individuals found that non-autistic people showed meaningfully larger pupils when viewing social stimuli compared to non-social stimuli, while autistic individuals showed no such difference between the two categories.16PubMed. Pupillary responses for social versus non-social stimuli in autism: A systematic review and meta-analysis This does not mean autistic people are not processing social information. It means the processing may not produce the same distinctive autonomic arousal signature, or that attentional allocation is organized differently.
Social anxiety also changes the picture. Research on children with social anxiety disorder found a blunted pupillary reactivity to emotional faces. Girls with social anxiety showed reduced pupil dilation in response to happy and angry faces, while boys with social anxiety showed reduced dilation to neutral faces. Across both sexes, the degree of reduced dilation correlated with the severity of social anxiety.17PubMed. Children with social anxiety disorder show blunted pupillary reactivity and altered eye contact processing in response to emotional faces One interpretation is that anxiety-driven avoidance dampens the engagement signal before it can fully develop. These children are not less affected by faces; they may be so threatened by them that their processing shuts down rather than ramping up.
Pupil Mimicry Starts in Infancy
Pupillary contagion is not something you learn. Infants as young as four to six months old show the effect. When babies viewed images of eyes with large pupils, their own pupils dilated more than when they viewed eyes with medium or small pupils, and this pattern did not appear for non-face geometric shapes.18PubMed. Pupillary Contagion in Infancy: Evidence for Spontaneous Transfer of Arousal The specificity to eyes suggests that babies are tuned to the social signal from very early in life, not just responding to a general brightness difference.
The developmental trajectory of this ability is also revealing. A study on five- to eight-month-old infants found that the youngest infants (five to six months) showed pupil contagion for both familiar-race and unfamiliar-race faces, as well as for upright and inverted faces. By seven to eight months, the contagion effect was more selective: it appeared for upright unfamiliar-race faces but not inverted ones, suggesting that face-processing specialization was beginning to narrow the conditions under which automatic mimicry occurs.19PubMed Central. The other-race effect of pupil contagion in infancy In other words, the raw capacity to catch someone else’s arousal through their pupils appears to be something humans are born with, and experience shapes how it is deployed.
Pupil Mimicry Across Species
Humans are not the only primates who mirror each other’s pupil size. When chimpanzees viewed images of eyes with dilating or constricting pupils, they mimicked the pupil changes, but only for conspecifics. Chimpanzees did not mimic the pupils of human eyes, and humans showed stronger mimicry for human eyes than for chimpanzee eyes. The effect was strongest among chimpanzee mothers, which the researchers interpreted as consistent with the idea that pupil mimicry serves a social bonding function.20PubMed Central. Chimpanzees and humans mimic pupil-size of conspecifics
The species-specificity of this effect suggests it is not a generic visual reflex but a social mechanism tuned to one’s own kind. The researchers speculated that pupil mimicry may have become more important in human evolution as the visible white sclera of human eyes made pupil changes easier to detect. Most other primates have dark sclera, which camouflages pupil movement. Our eyes may have evolved to make this social signal more readable, which in turn would have favored a stronger mimicry response. If that is right, the experience of feeling “drawn in” by someone’s gaze is not just a metaphor. It is the product of a feedback loop shaped by millions of years of social primate life, refined in a species whose eyes are built to broadcast what the brain is feeling.
Drugs, Darkness, and Other Confounds
It is worth noting that pupil dilation has many causes beyond emotional and social arousal. Anything that activates the sympathetic nervous system will dilate pupils, including stimulant drugs, pain, physical exertion, and simply being in a dark room. Some recreational substances produce dramatic dilation as a direct pharmacological effect: MDMA, for instance, produces substantial sympathetic activation at typical recreational doses, with measurable increases in heart rate, blood pressure, and subjective feelings of closeness to others.21PubMed Central. Physiological and subjective responses to controlled oral 3,4-methylenedioxymethamphetamine administration The pupil dilation users experience is driven by this surge in sympathetic activity, not by any special social circuitry.
This matters for the “romantic dilation” story because it means that context determines interpretation. If you notice someone’s pupils are wide while they’re looking at you in a candlelit room, the darkness alone could account for most of the change. If they’ve had coffee, their sympathetic baseline is already elevated. If they’re nervous about the situation rather than attracted to you, the dilation looks the same. The human eye cannot distinguish between “dilated because of you” and “dilated because of caffeine, dim lighting, and mild anxiety.” The signal is real at a population level in controlled experiments, but extracting reliable meaning from one person’s pupils in one moment is something even researchers struggle with. Treating pupil size as a mind-reading tool in daily life is a recipe for overconfidence.