Criticism from some scientists centered on the fact that while publications often appeared in the popular press about Koko, scientific publications with substantial data were fewer in number.[32][33][34] Other researchers argued that Koko did not understand the meaning behind what she was doing and learnt to complete the signs simply because the researchers rewarded her for doing so (indicating that her actions were the product of operant conditioning).[35][36] Another concern that has been raised about Koko's ability to express coherent thoughts through signs is that interpretation of the gorilla's conversation was left to the handler, who may have seen improbable concatenations of signs as meaningful. For example, when Koko signed "sad" there was no way to tell whether she meant it with the connotation of "How sad." Following Patterson's initial publications in 1978, a series of critical evaluations of her reports of signing behavior in great apes argued that video evidence suggested that Koko was simply being prompted by her trainers' unconscious cues to display specific signs, in what is commonly called the Clever Hans effect.[37][38][39][40][30][41]

Quelle: https://en.wikipedia.org/wiki/Koko_%28gorilla%29

A couple obvious problems present themselves when one looks into this talking-ape business. The first, as you suggest, is that interpretation of the gorilla’s conversation, if such it be, is left to the handler, who generally sees any improbable concatenation of signs as deeply meaningful. During the 1998 on-line chat you saw bits of in Harper’s (the whole thing is at www.koko.org/world/talk_aol.html) [Editor’s Note: archived page], for example, Koko, without being prompted or questioned, made the sign for nipple, which Francine Patterson, her trainer, interpreted as a rhyme for “people.” (Patterson further claimed that this was a reference to the chat session’s audience.) Even if you buy the idea that gorillas, who cannot speak, grasp the concept of rhyme, this sounds like wishful thinking. Similar examples abound: “lips” is supposedly Koko’s word for woman, “foot” her word for man. Koko made a lot of signs, and sometimes expressed desires or other thoughts, but nothing in the transcript suggests a sustained conversation, even of the simple sort you might have with a toddler.