THE CLEANER WRASSE charged toward the unknown fish, ready for attack. The stranger met her head-on. When she retreated, the stranger did too. In due time, she began darting, flipping upside down, and “dancing.” Eventually, she approached more slowly, turning her blue body sidewise, then sank, all the time watching.
What appeared to be a stranger was, in fact, her reflection peering back. Days after the first encounter, biologist Masanori Kohda placed his small research subject in a solution of eugenol, an anesthetic made from clove oil, and injected brown dye under the skin of her throat—a spot she couldn’t see directly. When, later, the wrasse again looked in the mirror, she swam to the bottom of her tank and began to scrape her marked scales against a rock. In nearby tanks, other fish that had been similarly marked did the same.
Kohda and his colleagues concluded that the fish had recognized their reflections, a claim that sparked an uproar when the team from Osaka City University released the findings in 2019.
The study made the cleaner wrasse the latest to join an expanding and controversial pool of animals reported to share a capacity once thought uniquely human. Great apes, dolphins, elephants, certain birds, manta rays, and, most contentiously, ants had all reportedly passed the mirror mark test, the standard gauge of self-awareness in animals since 1970.
The test itself is appealingly straightforward. Mark an animal somewhere it can see only with the aid of a mirror. If the animal then touches or investigates the mark on its own body, rather than on the mirror’s surface, it has recognized itself.
But recognition, scientists had long held, required a self-concept onto which the reflected image could be mapped. And that faculty was widely believed to be absent in most animals. They might experience color, pain, and pleasure, the thinking went, but without any sense of the self having the experience. They simply didn’t have the brains for it.
After the fateful 2019 study, however, the wrasse went on to pass multiple variations of the mirror mark test, accumulating evidence that now surpasses that for any other nonhuman animal.
Many researchers who had previously accepted the mirror test as the standard measure of self-awareness now began to look at it askance. The tiny fish had unwittingly shaken faith in a hypothesis the test’s creator had defended for half a century and whose root reached back further still: that self-awareness had developed just once in the animal kingdom.
Reflections and Refractions
Until the 15th century, even the well-to-do in Europe lived and died without ever clearly seeing their reflections. Older, imperfect mirrors, made of copper, bronze, and even silver, were found in courts and palaces beyond the view of ordinary people. In the early 1400s, convex glass mirrors began reaching the most affluent European households. As the technology spread, people became increasingly aware of how they appeared in the eyes of others.
Before glass mirrors, “individuality as we understand it today did not exist: people only understood their identity in relation to groups—their household, their manor, their town or parish—and in relation to God,” writes historian Ian Mortimer. “The very act of a person seeing himself in a mirror . . . encouraged him to think of himself in a different way. He began to see himself as unique.”
The impact on self-perception rippled through art, religion, and culture, Mortimer argues. Self-portraits became more common, and people began framing their relationship with God in more personal terms.
Not everyone trusted the reflection in the glass mirror, or what it represented. In his 1576 satire, The Steel Glass, poet George Gascoigne champions older, rough mirrors for seeing through vanity and deception and returning a more honest reflection. Newton, too, put his faith in the old technology. When he completed his reflecting telescope in 1668, it used a curved alloy mirror to gather starlight, a heavenly glimpse, he believed, of God’s design.
By the 1800s, divine providence was falling out of favor across the sciences. Still, when Western researchers studying the developing mind tried to pinpoint what made the human mind unique, it was the mirror they turned to.
In The Descent of Man, published in 1871, Charles Darwin laid out a theory of how human language came to be. At that time, language was widely considered humankind’s distinguishing ability—one that was not just more advanced than animal communication but categorically distinct. Darwin had a different view. For him, language development, like species evolution, would have happened incrementally. Seen this way, human speech was clearly on a continuum of animal communication.
In The Mirror and the Mind, historian of science Katja Guenther argues that this claim had far-reaching political ramifications. For a century, Enlightenment thinkers had argued for democratic rights on the basis of rationality, a capacity enjoyed by commoner and aristocrat alike. Unlike animals, humans could override their instincts and act according to reason. Since human language was perceived, in this view, to be the foundation of rational and ethical thought, placing it in lineage with the communication tools of other animals put the ordinary man’s claim to political freedoms on shaky ground.
Fortunately for philosophers, Darwin planted a new seed of human exceptionalism in 1877 when he recorded the contrasting responses of his infant son and higher apes when they looked in the mirror. Where the boy smiled, the apes grew angry and looked away.
Inspired by this observation, physiologist Wilhelm Preyer began observing his own young son’s responses to his reflection, part of research that would help found the field of child psychology. Preyer accepted Darwin’s contention that animals had some capacity for abstraction. A dog, for example, clearly understood certain words. But, he argued, only in humans did it culminate in introspection and, ultimately, self-awareness. When he saw his son begin to smile self-consciously and play dress-up in front of the mirror, Preyer believed he had found evidence of the ichbegriff (self-concept) emerging, a development he thought paralleled the evolutionary departure of humans from other animals.
For scholars looking to preserve human uniqueness without leaning on theology, it was an attractive theory. By granting humans individuality along with a unique capacity for higher thinking, Guenther argues, mirror recognition reinstated the divide between man and beast in a way that “flattered humans and endorsed the image they had created for themselves.”
A Mark of Genius
But when exactly did self-recognition occur? Even Preyer couldn’t say. In a letter to the American educator Emily Talbot, he acknowledged that pinning down the moment was “one of the most intricate questions to decide.” Self-consciousness, he wrote, “does not arise suddenly, but by degrees.”
One observer placed recognition at 12 months, another at 21, another not until 28. In the 1890s, developmental psychologist Milicent Shinn tried improving Preyer’s method by pooling hundreds of observations from a network of mother-researchers, but the ambiguity of mirror responses remained. Could one say for sure that an infant smiling in the mirror before turning away shyly believed it was looking at itself?
By the early 1900s, this subjective approach ran afoul of efforts to fold psychology into experimental science. Behaviorism, as the movement came to be known, demanded rigorous methodology and measurable results. As Guenther notes, the mirror offered neither.
While behaviorists rejected the very idea of mental states and, along with it, self-awareness, the notion that self-awareness set humans apart persisted among philosophers and developmental psychologists. And around 1970, two researchers, unaware of the other’s work, arrived at the same solution to the interpretability problem. One was developmental psychologist Beulah Amsterdam, who marked infants’ noses with rouge to see whether they would reach for their own faces. But it was a young comparative psychologist’s version of the test that would prove more consequential.
Gordon Gallup studied chimpanzee behavior and had been testing whether a mirror could stand in for another animal—a way to gauge a chimp’s social responses without needing a real partner on hand. Housed alone, each of his four chimps treated their reflection as a stranger at first, threatening it and calling out. After a few days, their behavior changed: they began using the mirror to groom themselves, pick food from their teeth, and examine parts of their bodies they had never seen. It looked like self-recognition. To confirm it, Gallup had the now-famous idea to mark them surreptitiously.
He anesthetized his chimps and marked one eyebrow and one ear on each with an odorless red dye. When subjected to the mark test, each of Gallup’s four chimps touched the dried dye with their fingers, one even bringing its hands to its nose to smell it. It was a monumental moment. He concluded that the chimps possessed self-awareness, which he later defined as the ability to become the object of one’s own attention.
The fact that the chimps could recognize themselves had broad implications, in Gallup’s view. He argued that self-awareness implied the ability to reflect on one’s goals, to think about oneself in the past, and imagine oneself in the future. And this ability produced, as a by-product, theory of mind—the awareness that other individuals hold beliefs and thoughts different from your own.
Directly afterward, he repeated his experiment with stump-tailed, rhesus, and crab-eating macaque monkeys. In his classic 1970 paper outlining this work, Gallup reported that, unlike the chimps, the monkeys treated their reflections the same way they would strangers. Monkeys, he concluded in the language of the time, were mindless.
“Our data suggest that we may have found a qualitative psychological difference among primates, and that the capacity for self-recognition may not extend below man and the great apes,” he wrote.
On one reading, his research cracked human exceptionalism; on another, it widened the circle just enough to include the other great apes, with whom we share a common ancestor. The idea came to be known as “the big bang theory” of self-awareness. By defining mirror self-recognition as mark exploration, a distinct, measurable act, Gallup, who died in July 2026, had found a way to bring self-awareness into the methodological parameters of behaviorism while reinforcing the scientific case for human exceptionalism.
The Circle Widens
B. F. Skinner, arguably the most well-known behaviorist of the time, was not impressed. The mirror mark test might have made mirror responses measurable, but it did not solve the problem of interpretability that mirror self-recognition had always run into. In his view, actions that appeared self-conscious were actually, like all behavior, learned responses to environmental conditioning. Actions followed by rewards become more frequent; those followed by punishments decrease.
To illustrate this point, Skinner and his colleagues taught three pigeons to use a mirror to locate and peck at marks on the walls of their enclosure by rewarding them for doing so. They then marked the pigeons’ chests, concealing the dots from their view with a bib. Using the mirror, the pigeons were able to locate and peck at the marks on their own bodies. The researchers published the study in 1981 with the ironic title, “ ‘Self-Awareness’ in the Pigeon.”
