![]() ![]() 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. The problem with this method is it assumes intelligence develops as a function of age in a linear way, which is an oversimplification. 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. ![]() What adult IQ does a mental age of 2.5 equate to? The question is a lot trickier than it seems. In other words, chimps have the same intelligence as a 2.5 year old (white) human. With the exception of social intelligence, where the human toddlers were way ahead, the apes and toddlers had the same intelligence. 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. 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) 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 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. 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! 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. The average chimp has a cranial capacity of about 400 cubic centimeters. Before investigating the IQ of chimps, it’s interesting to ask what the expected IQ chimps should have given their brain size. ![]()
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