In his 2006 book, Richard Lynn said chimpanzees “have a vocabulary of around a dozen cries to convey information,
including the presence of predators, intrusion into their territories of neighboring groups, the location of a supply of food, willingness or unwillingness to share food, and so on.”
Using my formula for equating total vocabulary to verbal IQ (Pumpkin Person 2021) a vocabulary of 12 equates to a verbal IQ of 19 (U.S. norms).
Verbal IQ = 0.002(vocabulary) + 19.35827
One problem with this estimate is that humans are socialized by other humans, and thus exposed to far more words than chimps are. A more accurate test of ape ability comes from studies of apes raised by humans. One such ape was Nim Chimpsky who was raised from infancy by humans in an attempt to debunk Noam Chomsky’s theory that only humans can use language.
While Nim did learn 125 signs, Terrace concluded that he had not acquired anything the researchers were prepared to designate worthy of the name “language” (as defined by Noam Chomsky) although he had learned to repeat his trainers’ signs in appropriate contexts. Language is defined as a “doubly articulated” system, in which signs are formed for objects and states and then combined syntactically, in ways that determine how their meanings will be understood. For example, “man bites dog” and “dog bites man” use the same set of words but because of their ordering will be understood by speakers of English as denoting very different meanings.
One of Terrace’s colleagues, Laura-Ann Petitto, estimated that with more standard criteria, Nim’s true vocabulary count was closer to 25 than 125. However, other students who cared for Nim longer than Petitto disagreed with her and with the way that Terrace conducted his experiment. Critics[who?] assert that Terrace used his analysis to destroy the movement of ape-language research. Terrace argued that none of the chimps were using language, because they could learn signs but could not form them syntactically as language.
So raised by humans Nim had a vocabulary anywhere from 25 to 125, which in my formula equates to a verbal IQ of 19 to 20 (the same as chimps in the wild).
More impressive claims are made for Koko the gorilla which is surprising because although gorillas have bigger brains than chimps, they are less encephalized and more genetically distant from humans.
Her instructor and caregiver, Francine Patterson, reported that Koko had an active vocabulary of more than 1,000 signs of what Patterson calls “Gorilla Sign Language” (GSL). This puts Koko’s vocabulary at the same level as a three-year-old human. In contrast to other experiments attempting to teach sign language to non-human primates, Patterson simultaneously exposed Koko to spoken English from an early age. It was reported that Koko understood approximately 2,000 words of spoken English, in addition to the signs.
2000 words equates to a verbal IQ of 23. But if humans use words in qualitatively superior ways than apes do (syntax) then vocabulary might overestimate ape verbal IQ, because even when humans and apes are matched on vocabulary, the human can put the words in much more meaningful order. On the other hand, it’s largely because apes can’t grasp syntax that their vocabularies stagnate in the first place, so perhaps this measure is reasonable.
Another excuse to play one of my favorite bands: