In 1956, a trio of social scientists—Donald Campbell and Melville Herskovits from Northwestern University and Marshall Segall from Syracuse University—embarked on an ambitious journey to answer one of psychology’s most tantalizing questions: Do people from different cultures actually see the world in the same way? Their quest, which spanned continents and cultures, would ultimately reshape how researchers think about the very nature of human perception.
Their project took them from the urban depths of a gold mine in Johannesburg to the remote foraging communities of the Kalahari Desert, from the Philippine island of Mindoro to the collegiate halls of Evanston, Illinois. Everywhere they went, they carried a simple but powerful tool: a booklet filled with optical illusions, most notably the Müller-Lyer illusion. This illusion, familiar to many, consists of two identical horizontal lines—one capped with inward-facing arrowheads, the other with outward-facing ones. To most viewers, the line with inward-pointing arrows appears longer, even though both are exactly the same length.
When the results of their study emerged in 1961, the psychological community was stunned. According to reporting from Slate, American students in Illinois almost universally saw the top line as longer. Zulu pastoralists in South Africa, however, were far less susceptible to the illusion, and the San foragers of the Kalahari seemed to see nothing unusual at all—just two lines of equal length. It was as if the very act of seeing could be fundamentally different depending on where a person grew up.
This finding sent ripples through the field of psychology, which had long relied on a narrow subject pool—mostly college students from Western, Educated, Industrialized, Rich, and Democratic (WEIRD) societies. As The New York Times and other outlets have pointed out, this reliance raised uncomfortable questions about whether psychological findings could truly be generalized to all of humanity. The Müller-Lyer study seemed to offer a dramatic example: perhaps even our basic perceptions weren’t as universal as once believed.
Segall and his colleagues went further, proposing what became known as the Cultural Byproduct Hypothesis. They argued that people raised in environments filled with straight lines and angular structures—think carpentered cities and boxy buildings—would be more susceptible to illusions like Müller-Lyer. In contrast, those who grew up in natural, less structured settings would be less affected. This idea quickly became a staple of psychology textbooks, taught to generations of students as a cautionary tale against assuming universality in human experience.
But is the story really that simple? In a recent 2025 paper, a new group of researchers revisited more than a century of perceptual studies and arrived at a strikingly different conclusion. Their findings, as reported by Slate and others, challenge the notion that the Müller-Lyer illusion is merely a cultural artifact. Instead, they argue, the illusion and many other aspects of perception are likely hardwired into the human species.
What evidence supports this bold claim? For starters, the Müller-Lyer illusion isn’t unique to humans living in carpentered environments—or even to humans at all. Studies have shown that a variety of animals, from guppies to monkeys, horses, parakeets, and lizards, also fall for the illusion. As one researcher quipped, “Did the guppies’ culture create the illusion for them too? Seems unlikely.” The illusion also persists when the lines are replaced with curves, clusters of dots, or even faces, undermining the claim that it’s specifically tied to straight lines and right angles.
Perhaps the most compelling evidence comes from the Prakash project in North India. In this humanitarian and scientific effort, children born with congenital cataracts—who had never seen light, let alone buildings—received corrective eye surgery. Astonishingly, just hours after their first exposure to sight, these children reported seeing the Müller-Lyer illusion exactly as sighted people do. As the researchers noted, “Not only had these children never seen carpentry, they had never seen anything. And yet they still experienced the illusory effects of the figure.”
Given such strong evidence, why did the original cross-cultural studies suggest otherwise? The answer, it seems, lies in the messy realities of fieldwork. According to the recent analysis, earlier studies were riddled with inconsistencies and potential biases. Translation difficulties often made it hard to ensure that instructions were understood the same way across languages and cultures. Experimenters, sometimes unconsciously, may have nudged participants toward expected answers. In one telling admission, a researcher wrote, “I developed very strong expectations of what answer the respondents should give to a given item, and if a respondent gave the other answer, there was the impulse to correct the respondent to ask him to reconsider.”
Moreover, selective data exclusion may have played a role. In some cases, results that contradicted the prevailing narrative were omitted or downplayed, further muddying the waters. As the new paper points out, even within the original studies, there were contradictions: for example, mineworkers—who lived in highly structured, carpentered environments—showed some of the weakest susceptibility to the illusion, while some rural populations displayed unexpectedly strong effects.
So what does all this mean for the future of psychology? For one, it’s a reminder that broadening research to include diverse populations is both necessary and valuable. As the authors of the new study put it, “Expanding psychological research to capture the diversity of human experience is a tide that lifts all boats.” But it’s also a caution against assuming that every difference observed across cultures is a product of culture alone. Some aspects of perception, it seems, may be universal—rooted in our biology rather than our upbringing.
This doesn’t mean that culture is irrelevant. Far from it: cultural context shapes countless aspects of how we think, feel, and interact with the world. But when it comes to certain fundamental perceptual processes, the evidence increasingly points toward a common human experience—one we share not only with each other, but, remarkably, with a host of other animals as well.
As researchers continue to untangle the complex interplay between culture and cognition, the Müller-Lyer illusion stands as a testament to both the diversity and the unity of human perception. The question of how we see the world—once thought to divide us—may, in the end, reveal just how much we have in common.
