Two psychologists in the 1930s adopted a baby chimp and tried to raise her alongside their own baby boy, Donald, to see if this caused the chimp to learn human behavior. After nine months, they stopped the experiment because their son started behaving more like a chimp.
But eventually, as NPR notes, Gua hit a cognitive wall: no amount of training or nurturing could overcome the fact that, genetically, she was a chimpanzee.
As such, The Psychological Record authors write, the Kelloggs' experiment "probably succeeded better than any study before its time in demonstrating the limitations heredity placed on an organism regardless of environmental opportunities as well as the developmental gains that could be made in enriched environments."
Source: smithsonianmag.com
