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Before investigating the IQ of chimps, it’s interesting to ask what the expected IQ chimps should have given their brain size.

Chimp brain size predicts an IQ of 20

The average chimp has a cranial capacity of about 400 cubic centimeters.  The average cranial capacity of white adults in modern Western countries is not known because most of the cited figures come from people who lived before WWII and we know brain size like height has increased since then.  Perhaps the best estimate for modern white cranial capacity is 1474 cubic centimeters, since this is the cranial capacity of Eurasians before the malnutrition and disease of agriculture shrunk our brains and bodies, and with modern nutrition, we’ve hopefully recovered this lost brain size in the generations since WWII!

Given that the within sex standard deviation for cranial capacity among whites is about 91 cc, this implies the chimps are 11.8 standard deviations smaller in brain size than modern whites.  Given about a 0.45 within sex correlation between brain size and IQ in Western samples, this predicts their IQs would be 11.8 SD(0.45) = 5.31 SD below the white mean.  Given that the (white) American mean and SD for IQ are defined as 100 and 15 respectively, this equates to an IQ of 100 – 5.31(15) = 20 (white norms)

How do chimps score on “IQ tests”?

In 2007 there was a fascinating study that compared human 2.5 year-olds to chimps and other apes on a battery of intelligence tests.  With the exception of social intelligence, where the human toddlers were way ahead, the apes and toddlers had the same intelligence.

In other words, chimps have the same intelligence as a 2.5 year old (white) human.

What adult IQ does a mental age of 2.5 equate to?  The question is a lot trickier than it seems.  One could define adult mental age as 16+ and then use the age ratio method to conclude that since 2.5 is 16% of 16, a mental age of 2.5 equates to an adult IQ of 16.  The problem with this method is it assumes intelligence develops as a function of age in a linear way, which is an oversimplification.

What is needed is an actual intelligence test that’s been given to both adults and to toddlers and one where scores increase on an interval scale.

One such test is digit span.  Since the earliest days of intelligence testing (digit span has virtually no Flynn effect) it’s been known that by the age of three, a white child can repeat two digits, which probably means a 2.5 year old can repeat one digit.

By contrast U.S. adults average a forward digit span of 6.645 with a standard deviation of 1.35 and since races in the U.S. differ very little on forward digit span, this should be taken as the white adult distribution.  This means that an adult who performs like a 2.5 year-old (digit span of 1) is  4.18 standard deviations below the white adult average.

If we assume that most cognitive abilities are like digit span,  then chimps (who score like 2.5 years olds on most tested cognitive functions) perhaps average 4.18 standard deviations below white adults on the average test.

Does this mean their IQs average 4.18 SD below the average white adult?  No, because if you score 4.18 SD below white adults on the average test, your composite score on a battery of tests is actually much lower.  Why?  Because it’s much more rare to average an extremely low score across a battery of tests than it is  to score that low on any one test.  Indeed based on the intercorrelation of WAIS-IV subtests, someone who is 4.18 SD below average on the average subtest, would be 5.73 standard deviations (86 IQ points) below average on the composite score, thus my best guess for the average IQ of chimps is 14 (white norms).

This is remarkably close to the predicted IQ of 20 based on chimp brain size.  The fact that the IQ of a species separated from us by over 6 million years of evolution, can have their IQs so closely predicted by the same regression line predicting Western human IQ from Western human brain size, suggests that it’s possible to create culture reduced IQ tests that transcend not only culture, time and space, but species too.

IQ is measuring something very real and biological.