Book review – Why Chimpanzees Can’t Learn Language and Only Humans Can

7-minute read

The title of this book leaves little to the imagination and seems like a strong statement – how can we be so sure? The author, behavioural psychologist Herbert S. Terrace, is in a very strong position to make this claim though. Here, he revisits a remarkable experiment conducted in the 1970s to teach a chimpanzee to speak using sign language that ultimately failed. Bringing together subsequent developments in linguistics, palaeoanthropology, and developmental psychology, he has written an incredibly interesting and well-structured book on the evolutionary basis of language.

Why Chimpanzees Can’t Learn Language and Only Humans Can, written by Herbert S. Terrace, published by Columbia University Press in October 2019 (hardback, 219 pages)

The roots for Terrace’s story run back all the way to 1959 when a young upstart professor by the name of Noam Chomsky published a scathing review of the theory of language laid out in the book Verbal Behavior by the influential behavioural scientist B.F. Skinner (yes, biologists, he of the Skinner box). Without going into the details, what is relevant here was Chomsky’s claim that language was uniquely human and did not evolve from animal communication. This is where Terrace enters the story.

Terrace was hoping to prove Chomsky wrong and started Project Nim: a bold attempt to try and teach a young chimpanzee (cheekily named Nim Chimpsky) to use language. Where early attempts at teaching chimps to speak had failed because they cannot vocalise like humans, Terrace turned to American Sign Language. He details how Project Nim worked and why it failed. Although Nim learned to use signs, he did a Clever Hans on them (the infamous German horse that supposedly could do arithmetics but in reality picked up on subtle involuntary cues from his trainers). Careful analysis showed that Nim’s trainers were prompting him and that Nim only signed to earn rewards.

“Project Nim was a bold attempt to try and teach a young chimpanzee to use language. But he did a Clever Hans on them: careful analysis showed that Nim’s trainers were prompting him and that Nim only signed to earn rewards”

The first two chapters furthermore examine other ape language projects that used visual symbols, or lexigrams, to teach apes to speak, most famously Kanzi. Terrace concludes that these, too, showed apes capable of learning tricks (combining sequences of lexigrams to obtain rewards), but not of understanding their meaning. But what of other animals, you might ask? Although this book focuses on primates, he does shortly discuss experiments of word comprehension by dogs before pointing out that they do not imply (nor claimed) linguistic knowledge on the dogs’ part.

So, Terrace ended up agreeing with Chomsky that language is uniquely human, but he continues to disagree with him on how language evolved. In case you are wondering why this Chomsky keeps coming up, just a quick aside. From abovementioned young upstart professor, Noam Chomsky became one of the most influential linguists whose quest for a Universal Grammar has shaped linguistics over the last five decades. This research programme proposes that there exists a set of innate rules that can be applied to each and every language to create meaningful sentences.

“In the decades since Project Nim ended, we have found fossil remains of many human ancestors that evolved after the split from chimpanzees. But which of these used language?”

Now, before you think that Terrace has a chip on his shoulder, that is not the case. Throughout the book, he expresses his admiration for Chomsky’s quest. Nevertheless, he is also critical of some aspects of it, though, as elsewhere in the book, in an insightful and respectful fashion. For one, Chomsky’s focus on grammar ignores the origin of words, which Terrace argues had to be in place before grammar could even evolve. Or, as he puts it on page 141: “Chomsky’s neglect of words leaves us with half a loaf“. Second is that it provides little insight into the evolution of language. Recently, Chomsky has settled on what arguably seems a bit of a cop-out solution: grammar evolved thanks to a single mutation some 80,000 years ago that produced a “slight rewiring of the brain” (see my review of Some Assembly Required why this may be less of a cop out than you think). Terrace envisions a different solution to the evolution of language and dedicates two fascinating chapters to findings from palaeoanthropology and developmental psychology.

In the decades since Project Nim ended, we have found fossil remains of many human ancestors that evolved after the split from chimpanzees. But which of these used language? Here he follows Derek Bickerton, who proposes that Homo erectus was that species. They had a significantly larger brain than others, requiring a higher intake of calories to fuel this energy-hungry organ. Lacking weapons, this required them to scavenge for meat, providing a strong selective pressure for language to communicate the discovery of a carcass. There is interesting evidence here from the order in which cut and bite marks were laid down on bones that suggests hominins started having first dibs on carcasses around this time (Agustín Fuentes also noted this, arguing for a switch from passive to power scavenging about 2 million years ago). It is a neat idea, although it does not address why H. erectus evolved a larger brain to begin with. Is this another just-so story? Botha’s Neanderthal Language also highlights collective hunting behaviour as a driver of language in this more recent hominin, so maybe my scepticism is misplaced. It is pleasant to see that Terrace never overstates his case, acknowledging and welcoming what future discoveries will add to our knowledge.

“The other strand of evidence Terrace draws on is modern developmental psychology. The nonverbal relations between a mother and her infant during its first year are crucial to the development of language.”

The other strand of evidence Terrace draws on is modern developmental psychology. The nonverbal relations between a mother and her infant during its first year are crucial to the development of language. Cradling, too, has deep evolutionary roots. The switch to bipedalism reduced the size of the birth canal. As brain size increased, babies were born not yet fully developed and required months of cradling and protection. Something which, Sarah Hrdy argues in Mothers and Others, did not just involve the mother, but also other group members. This form of cooperative breeding, not seen in other primates, could both have profited from and contributed to increased communication, probably being inseparably intertwined with it.

The only thing that you will not find in this book is a reflection on the ethics of Project Nim, other than an admission that the National Institutes of Health nowadays prohibits research on chimpanzees. The reason I bring this up is that you might have heard of it through either the book Nim Chimpsky or the 2012 documentary Project Nim. In the epilogue, Terrace vents his frustration with the documentary, which he feels misrepresents him, but, more importantly, fails to present the scientific background to the research.

Why Chimpanzees Can’t Learn Language and Only Humans Can is an incredibly interesting book that manages to cover an amazing amount of material in only 178 pages. It is also well-structured, with each chapter ending with a very helpful summary, greatly enhancing understanding of the text. If the title even slightly tickles you, then do not give this book a miss.

Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however

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