Why is it difficult to tell whether an ape’s series of signs follows rules like a human sentence?

It is difficult to determine if an ape’s series of signs follows human-like syntactic rules because of the challenge in distinguishing between genuine linguistic communication and learned instrumental responses. Skeptics argue that when an ape produces a string of signs, it may simply be performing an arbitrary sequence of actions to obtain a reward, similar to how a pigeon can be trained to press keys in a specific order.1

Several factors contribute to this uncertainty:

• Interpretation Bias: Observers may provide an overly rich, anthropomorphic interpretation of an ape's behavior, whereas the animal might merely be responding to cues from its trainer.1
• Trainer Influence: Research suggests that a high proportion of an ape's gestures may be direct imitations of the trainer, or that each sign in a sequence is prompted individually by the trainer rather than being part of a self-generated sentence.1
• Experimental Constraints: Studies that prioritize rigorous control often rely on reward-based training, which may prevent the ape from using signs to communicate shared interests or comments as a human child would.1

Ultimately, proving linguistic ability requires demonstrating that an ape understands how changing the order of signs alters their meaning in predictable, generalizable ways.1