In 1931, psychologist Winthrop Niles Kellogg and his wife Luella brought a seven-and-a-half-month-old female chimpanzee named Gua into their home to raise alongside their ten-month-old son, Donald. For nine months, the couple treated both infants identically, feeding, clothing, and testing them in parallel to see whether a chimpanzee raised in a fully human environment could develop human-like abilities. The experiment produced a trove of developmental data, some genuine surprises about how quickly a young chimp could outpace a human infant on certain tasks, and one deeply unsettling discovery that helped bring the whole project to an early end.
Why the Kelloggs Launched the Study
Winthrop Kellogg was a comparative psychologist at Indiana University with a keen interest in the nature-versus-nurture debate. By the late 1920s, behaviorism dominated American psychology, and its central claim was bold: environment, not heredity, was the primary sculptor of behavior. Kellogg wanted to test how far that principle could stretch across species. If you gave a great ape the same rich, human rearing environment that a child receives, how closely would the ape’s development track a human child’s? Previous attempts to raise apes in human settings had been informal and anecdotal. Kellogg wanted something rigorous.
The opportunity came through the Yerkes Laboratories of Primate Biology in Orange Park, Florida, where Kellogg had a professional connection. A young female chimpanzee born in captivity in November 1930 was made available for the project. Kellogg and Luella agreed to raise the chimp alongside their own son. Donald was born in August 1930, making the two subjects close in age. On June 26, 1931, Gua arrived in the Kellogg household, and the experiment officially began.
How the Experiment Was Run
The design was straightforward but demanding. Gua was to be treated as a human child in every respect, never as a pet or a laboratory animal. She wore clothes, sat in a high chair, was kissed goodnight, and received the same affection, discipline, and daily routines as Donald.1OSF Preprints. Revisiting The Ape and The Child Experiment: A Mindsponge Approach The Kelloggs kept meticulous day-by-day records that included written observations, photographs, home movies, and structured tests administered at regular intervals.
The tests covered a wide range of abilities. The Kelloggs measured reflexes, motor coordination, problem-solving, memory, responsiveness to verbal commands, food preferences, emotional reactions, social behavior, and attempts at communication. Both subjects were tested under the same conditions, at the same ages, and their results were compared directly. The result was one of the most detailed developmental datasets of the early twentieth century.
Where Gua Outperformed Donald
One of the experiment’s most striking findings was that Gua initially surpassed Donald in a number of developmental milestones. She learned to respond to spoken commands faster than the boy did. When the Kelloggs said “close the door” or “shake hands,” Gua obeyed more quickly and consistently. She was also ahead in tasks requiring physical coordination and strength, which reflected the faster rate at which chimpanzees mature physically compared to humans during the first year of life.
Gua was quicker to learn to use a spoon, faster to respond to simple verbal instructions, and more adept at tasks that required manipulation of objects. She also displayed more sophisticated social awareness in some situations, reacting to the emotional states of the adults around her and adjusting her behavior accordingly. In the earliest months of the co-rearing period, an observer might reasonably have concluded that the chimpanzee was the more advanced of the two.1OSF Preprints. Revisiting The Ape and The Child Experiment: A Mindsponge Approach
Where the Chimp Plateaued
The advantage did not last. As weeks turned into months, Gua’s rate of learning slowed while Donald’s accelerated. Problem-solving tasks that required flexible thinking, combining steps, or understanding abstract relationships increasingly favored the human child. Gua could learn specific trained responses but struggled to generalize them to new situations in the way Donald began to do around his second year.
The most dramatic divergence was in language. Gua never produced anything resembling human speech. She could understand a modest number of spoken commands and phrases, roughly comparable to a well-trained dog, but she could not form words. Her vocalizations remained limited to chimpanzee-typical sounds: food barks, distress cries, and a small repertoire of emotional grunts. Despite being immersed in spoken English from early infancy, her vocal anatomy and neural wiring simply could not support the production of human language. This was the clearest evidence that some developmental trajectories are constrained by biology in ways that no amount of environmental enrichment can overcome.1OSF Preprints. Revisiting The Ape and The Child Experiment: A Mindsponge Approach
The Problem With Donald’s Speech
Here is the part of the story that tends to catch people off guard. While Gua never learned to talk like a human, Donald started sounding more like a chimpanzee. By the later months of the experiment, Donald had begun imitating Gua’s vocalizations. He used a chimpanzee-like food bark when he was hungry instead of words. At an age when children in typical environments are rapidly expanding their vocabularies, Donald’s spoken language was noticeably lagging behind developmental norms.2OSF Preprints. Revisiting The Ape and The Child Experiment: A Mindsponge Approach
This was alarming to the Kelloggs. The experiment had been designed to see if a chimp could be humanized, but the unintended consequence was that their son was being, in a sense, “chimpanzee-ized.” Donald was spending his most critical language-acquisition period in close daily contact with a companion whose primary mode of communication was nonverbal. Children learn language largely by imitating the communicative beings around them, and Gua was, from Donald’s perspective, his most constant and engaging social partner. The chimpanzee’s influence on the child’s development had not been adequately anticipated.
Why the Experiment Ended Early
The Kelloggs had originally planned to continue the co-rearing for a longer period, but Donald’s language delays forced their hand. In March 1932, after nine months, they terminated the experiment and returned Gua to the Yerkes primate colony in Florida. Donald’s speech recovered once he was no longer in daily contact with the chimpanzee, and he went on to develop normally.
The decision to end early was not made lightly. Kellogg had invested years of planning in the study and genuinely believed the data being generated were scientifically valuable. But the welfare of his child took precedence. The case has since become one of the most cited examples in psychology of an experiment that generated important findings partly through an outcome that was never intended.
What Happened to Gua
Gua’s story after the experiment is brief and sad. She was returned to the Yerkes Laboratories, where she lived among other captive chimpanzees. Reports indicate that she died of pneumonia in 1933, less than a year after the experiment ended. She was roughly three years old. Whether the transition from a warm domestic environment to institutional captive housing contributed to her decline is impossible to know for certain, but the shift from being dressed in human clothes and held by a human family to living in a primate colony was undoubtedly a profound change for an animal that had been reared to regard humans as her social group.
The Kelloggs themselves published their findings in a 1933 monograph titled The Ape and the Child: A Study of Environmental Influence Upon Early Behavior. The book presented their observations in meticulous detail and became a landmark text in comparative and developmental psychology.
What the Experiment Actually Showed About Nature and Nurture
The Kellogg study is sometimes described as proving that “nature wins,” but that is an oversimplification. What the experiment demonstrated is that both biological constraints and environmental inputs shape development, and the two interact in ways that are not always predictable.
On the “nature” side, Gua’s inability to acquire spoken language despite total immersion in a human-speaking household showed that chimpanzee vocal anatomy and neural architecture impose hard limits. No amount of nurture could give Gua the laryngeal control, the neural circuits for phonemic production, or the recursive grammatical capacity that human speech requires. These are species-level biological traits, not products of individual learning history.
On the “nurture” side, Gua’s early superiority in responding to commands, her capacity for emotional bonding with humans, and her adoption of many human-like behaviors (eating with utensils, responding to her name, expressing affection through hugging) showed that the environment could push a chimpanzee well beyond what would be observed in a colony-reared ape. The human rearing environment genuinely expanded her behavioral repertoire, even if it could not make her human.
And then there was the most surprising lesson, the one about Donald. The experiment revealed that environmental influence runs in both directions. If a human child is immersed in a social world that includes a chimpanzee as a primary companion, the child’s development can be pulled toward the chimpanzee’s communication patterns. This was not a trivial finding. It underscored that human language development depends not just on having a human brain but on having a human linguistic environment during the critical window for speech acquisition.
Ethical Questions That Have Grown Over Time
The Kellogg experiment was considered bold but scientifically acceptable in the 1930s. Today, it raises ethical red flags on multiple fronts. For the chimpanzee, being raised in a human environment and then abruptly returned to a primate colony is the kind of radical disruption in attachment and social bonding that modern animal welfare standards would prohibit. We now understand that great apes form deep emotional attachments, and severing those attachments causes lasting distress.
For Donald, the experiment placed a human child in a developmental environment that measurably delayed his language acquisition. Modern institutional review boards would almost certainly reject a study design that uses the researcher’s own infant as an experimental subject, particularly one where the intervention carries known developmental risks. The fact that Donald recovered fully does not erase the fact that the risk was taken without the kind of ethical oversight that contemporary science demands.
These concerns are not just retrospective moralizing. They reflect genuine shifts in how science weighs the value of knowledge against the cost to the subjects who produce it. The Kellogg experiment sits alongside other mid-twentieth-century studies that generated important data through methods that are no longer considered justifiable.
Later Cross-Fostering Studies and How They Built on Kellogg
The Kellogg experiment opened a door that several later researchers walked through, each time with different methods and different questions. In the 1960s and 1970s, Allen and Beatrix Gardner raised a chimpanzee named Washoe and taught her American Sign Language rather than trying to elicit spoken words. This sidestepped the vocal-anatomy barrier that had doomed Gua’s language prospects. Washoe eventually acquired over a hundred signs and could combine them in short phrases, a result that would not have been predicted based on Kellogg’s findings alone.
Other projects followed. Francine Patterson worked with Koko, a western lowland gorilla, using a modified sign language. Sue Savage-Rumbaugh worked with bonobos, particularly a male named Kanzi, who learned to communicate using a keyboard of abstract symbols called lexigrams. Kanzi’s comprehension of spoken English was tested rigorously and found to be comparable in some respects to that of a two-and-a-half-year-old human child.
Each of these studies owed a conceptual debt to the Kellogg experiment. Kellogg’s work had established two crucial baselines: first, that apes could learn a great deal from human environments, and second, that spoken language was the wrong modality to test their communicative potential. The shift to sign language and symbol boards in later decades was, in part, a direct response to what happened with Gua. Research on primate social learning has continued to evolve, with recent work tracking how novel behaviors and traditions spread through entire groups rather than just between pairs of individuals.3Neuroscience & Biobehavioral Reviews. Social learning, culture and the ‘socio-cultural brain’ of human and non-human primates
Donald Kellogg’s Later Life
Donald Kellogg grew up to lead a life far from the spotlight of his father’s most famous experiment. After his language caught up in the months following Gua’s departure, he developed normally through childhood and adolescence. He pursued an education, eventually studying medicine. However, Donald’s life ended tragically. He died by suicide in 1973 at the age of 43. His father, Winthrop, died shortly afterward in the same year.
There is no scientific basis for drawing a causal line between the nine months Donald spent with Gua and the circumstances of his death four decades later. Human lives are shaped by countless factors, and attributing an outcome so distant in time to a single early experience would be speculative in the extreme. Still, the fact inevitably colors how the public remembers the experiment. It adds a layer of melancholy to a story that already carries notes of loss, both for the chimp who was removed from her surrogate family and for the child who was placed in an experiment he never consented to.
Winthrop Kellogg continued his career in comparative psychology for decades after the experiment, publishing on topics including echolocation in porpoises. The ape-and-child study remained his most widely known work, cited in textbooks across psychology, anthropology, and linguistics. It endures as a vivid case study in the power and limits of environmental influence, a reminder that raising a child and a chimpanzee under identical conditions does not produce identical outcomes, and that the effects of such an arrangement can flow in directions no one anticipated.