New research published in Cognition and Emotion provides evidence that subtle changes in eye appearance can shape how people perceive others and influence their generosity. Across two experimental studies, researchers found that portraits of non-human primates with larger pupils elicited more positive impressions and greater charitable donations. Additionally, these effects were amplified when the primates had brighter irises, enhancing the visibility of pupil size.
The eyes have long been recognized as a critical aspect of social interaction, influencing how we perceive and relate to others. Previous studies have shown that pupil size can signal emotions, interest, and trustworthiness. Larger pupils are often associated with positive traits such as friendliness and attractiveness, while smaller pupils may indicate disinterest or sadness. However, the role of iris brightness in enhancing these effects has remained largely unexplored.
“A lot of us understand that eyes are extremely important for communication, at least intuitively, and I’m no different,” explained study author Juan Olvido Perea-García, who conducted the research while affiliated with the CoPAN lab at Leiden University and the Center for Language Evolution Studies at Michael Copernicus University.
“I started studying more obvious communicative functions over ten years ago. At first, my research focused on gaze-following and how the contrast between the iris and the ‘whites’ of the eye allows us to discern the direction that someone is looking easily. Slowly, I expanded this research to look at how other parts of the eye may transmit information to onlookers. I am fascinated by eyes, communication, and evolution, so I guess it is a natural topic to look into.”
The study was carried out in two parts, with participants recruited from Leiden University. In the first study, 64 students viewed digitally altered photographs of 32 non-human primate species. These primates were carefully selected to represent the full diversity of the primate evolutionary tree, ensuring that the findings would apply broadly across the primate lineage. This approach also avoided an overreliance on particularly “cute” species, allowing the researchers to evaluate the effects of their manipulations on both familiar and less well-known primates.
Each portrait was manipulated to display either constricted or dilated pupils while keeping iris brightness constant. Participants were asked to rate the primates on cuteness, friendliness, and attractiveness and to decide how much money they would donate to conservation efforts for each primate, with amounts ranging from zero to twenty euros.
In the second study, 121 participants viewed similar photographs, but this time both iris brightness and pupil size were manipulated. The irises were digitally adjusted to appear either brighter or darker, and the same questions about ratings and donations were posed. This design allowed the researchers to explore how the combination of iris brightness and pupil size influenced participants’ responses.
To ensure methodological rigor, participants were tested individually via an online platform, with clear instructions to avoid distractions. The researchers also employed manipulation checks to exclude participants who detected the image alterations.
The results confirmed that larger pupils were consistently associated with more positive ratings of cuteness, friendliness, and attractiveness. In the first study, participants also donated more money to primates with dilated pupils, although the effect size was modest. Notably, the impact of pupil size varied across species, with greater effects observed for primates whose irises naturally make their pupils more conspicuous.
“The findings were surprising to me,” Perea-García told PsyPost. “I am very skeptical of previous studies that also found substantial effects of pupil size on trait attribution and impression formation, but these results checked my skepticism and are forcing me to be more open about this topic. It was also utterly unsurprising to see that almost all our participants invariably chose the bald uakari as the least ‘attractive’ of the species we showed.”
The second study provided further insights. Participants rated primates with brighter irises as cuter, friendlier, and more attractive, but only when the pupils were dilated. When the pupils were constricted, brighter irises led to less favorable ratings. Donations were highest for primates with bright irises and dilated pupils, suggesting that the combination of these traits enhances the perception of approachability and positive emotional states.
The findings highlight “how affected we are by seemingly insignificant features that we do not consciously perceive,” Perea-García said. “Here, we explored this in a context of donation to endangered primates, but we are prone to acting on appearances more than we are aware of. We should remain skeptical of first impressions and try to hold judgments as much as we can because, as hard as this may be to accept, we are irrational agents and we act on appearances more than we may be willing to inspect and admit.”
But as with all research, there are caveats to consider. The artificial manipulation of images, though carefully executed, may not fully replicate natural variations in eye appearance. Future research could explore these effects using live interactions or more dynamic stimuli, such as videos.
“The most important issue here is that we manipulated the appearance of pupils and irises in primate portraits beyond what may naturally occur (what is known as ‘hyper stimulus’),” Perea-García noted. “This is common in many studies and does not critically undermine their results, but it is important to bear in mind.”
Additionally, the study focused exclusively on non-human primates, which, while advantageous for reducing pre-existing biases, may not fully capture how people respond to human eyes. Investigating how these findings translate to human social interactions could provide a deeper understanding of the mechanisms at play.
“While the beginnings of my academic studies on eye color are around the theme of communication, I slowly realized a lot of the variation we see responds, rather, to the environmental lighting in which each primate species lives,” Perea-García said. “Primates live from up north (like Japanese macaques) to close to the equator (like black-crested macaques), and light in each of these environments is very different. Some parts of the eye of primates appear darker where it is sunnier, very much like skin in humans.”
“Ultimately, I would like to reconcile the multiple functions of eyes—communicative and visual. How the pressure to have organs optimized for vision is in tension with the pressure to inform members of the same species, and how these two pressures influence each other over evolutionary time.”
“Just for fun, I encourage readers to check out the supplementary files in the article,” Perea-García added. “These show rankings of individual species in our study on different dimensions. Look up each species and see if your rating would match the rating of our participants! For example, which is the cutest species we included? I bet it is either an ape or a callitrichid!”
The study, “Pupil size and iris brightness interact to affect prosocial behaviour and affective responses,” was authored by Juan Olvido Perea-García, Daisy Berris, Jingzhi Tan, and Mariska E. Kret.
