Though the Kelloggs had originally planned for a five-year study, the research project came to an abrupt end when they realized that Donald was being adversely affected by being raised along with Gua Not only did he begin giving guttural food barks similar to Gua when asking for food, but he also developed the alarming habit of grunting at his parents the way that his "sister" did. It was at this point, just nine months after their research began, that the Kelloggs decided that the experiment needed to be terminated.
As Winthrop Kellogg later wrote:
The situation in which the two lived together as playmates and associates was much like that of the two-child family in which Gua, because of her greater maturity and agility, played the part of the older child. With the added stimulation thus afforded, the younger child in such situations usually learns more rapidly than would otherwise be the case. It was Gua, in fact, who was almost always the aggressor or leader in finding new toys to play with and new methods of play; while the human was inclined to take up the role of the imitator and follower.
Still, by the time the Kelloggs terminated their experiment, Gua had made amazing progress. She had learned to walk upright and respond to over twenty different commands, including "Shake hands" and "Open the door" (Donald had only learned three by this point). Despite her progress however, Winthrop and Luella Kellogg had grave doubts about what was Gua was actually learning. As they later commented:
"Let us consider finally the ability of the subjects [apes] in the comprehension of language. We are here faced, as in many earlier instances, with the problem of inner mental processes, for we cannot tell in a strict usage of the term whether the subjects introspectively comprehend what is said about them or not. All we can do is to observe whether they are able to react distinctively and individually to separate words and phrases. This, then, must serve as our criterion of 'comprehension.'
Still, while Donald was outperforming Gua in many other ways, including the spontaneous use of language, Gua's progress could not be doubted. What she learned while raised as a human would shatter many of the assumptions previously made about the differences between humans and non-humans.
Which still raised the question of how successful Gua could be in becoming readjusted to life among other research chimpanzees after living most of her life separated from other of her own kind. The Kelloggs did their best to prepare Gua for her return to living with other chimpanzees at the primate centre in Florida. However, the transition was likely traumatic considering they were the only real parents Gua ever knew. I haven't been able to determine anything of Gua's life at the primate centre except that Robert and Ada Yerkes continued to use Gua in their own chimpanzee research. For whatever reason, Gua didn't live long after the Kelloggs returned to Indiana. She died of pneumonia on December 21, 1933, less than a year after being separated from her "brother" and the loving care she received with the Kelloggs. Her death so soon afterward represents a tragic (and little-mentioned) footnote in what is otherwise hailed as a classic study in human-ape communication.
As for Winthrop and Luella Kellogg, their 1933 book, The Ape and the Child, became an instant classic. One reviewer described it as a "well-written, mature, and scholarly piece of psychological literature." The reviewer went on to say that it should be "required reading in all courses where the problem of heredity and environment is discussed." Not surprisingly, the Kellogg study generated enormous publicity with stories about Donald and Gua being written in the New York Times and other popular newspapers. Video clips from their life together are still available online.
It's hard to say how much of an impact the Kellogg study actually had on the eugenics movement (which was already dying out in the United States due to revelations over what was happening in Nazi Germany) as well as the entire heredity vs. environment debate. Still, what Winthrop and Luella attempted to do inspired other researchers to try similar studies. During the 1950s, psychologists Keith and Catherine Hayes launched their own experiment in raising a chimpanzee named Viki as a human child. They even managed to train Viki to speak four English words after extensive speech therapy though the barrier to full human speech seemed impassable.
Of course, the most well-known experiment of its kind was Allen and Beatrix Gardner's work during the 1960s and 1970s teaching American Sign Language (ASL) to a chimpanzee named Washoe. Not only did Washoe manage to learn hundred of ASL words, she even taught some ASL to her own son, Louis. Though this work is still controversial, the Gardners' success with Washoe and other chimpanzees helped revive interest in animal communication as well as the biological roots of language. Similar work with a gorilla named Koko has reportedly led to her learning as many as one thousand words in a modified version of ASL. Again, the research results evaluating Koko's progress continues to spark controversy.
As for Winthrop Kellogg, he went on to become one of the pioneers in animal research for much of his long career as a psychologist. Having moved to Florida State University in 1950, he was instrumental in reshaping the psychology program there and also became part of Florida State's marine research laboratory. Though his later research would never be as famous as what he had tried to do with Gua, he also became one of the first researchers to study how bottlenose dolphins use sonar to navigate as well as exploring dolphin communication and problem-solving ability. This involved inventing a special dolphin pool for use in research as well as becoming a scuba diver himself to explore the research environment from the point of view of the dolphin. He would later attempt to apply what he had learned about echolocation in dolphins to help blind humans navigate.
Even in retirement, Winthrop Kellogg stayed active n research including directing graduate students looking at cognitive ability in sea lions. He died in the summer of 1972, ten years after formally retiring from teaching. His wife Luella died just a few months later and, tragically, their son Donald died in the following year.