A while ago I tried to estimate the IQ of chimpanzees and came up with a number so astronomically low that I dismissed the exercise as absurd. I began by noting that chimps have an average brain size of about 400 cc (cubic centimeters) while young adult white Americans have an average brain size of about 1395 cc. Within each sex, brain size correlates about 0.4 with IQ (0.44 corrected for reliability) and the within sex brain size standard deviation is 91 cc. What this means is that since diverging from chimps, brain size within each sex has increased by an astonishing 10.93 standard deviations.

Now part of this can be explained by the fact that white adult Americans are bigger than chimps, but even controlling for fat free body weight, their brains are still an astonishing 9.9 SD bigger than chimp brains.

Estimating chimp IQ from brain size alone

Now if we assume that the evolution of brain size was entirely caused by selection for greater intelligence, then the 0.44 correlation between IQ and brain size implies that chimps have an IQ that is 9.9 SD/0.44 = 22.5 SD below the white American mean. In other words, an IQ of -238.

Even if we assume that only half of the 9.9 SD increase in brain size was caused by selection for intelligence (perhaps the other half was for more advanced emotions and physical coordination), you still end up with a likely IQ of well below zero.  And complex emotions only evolve to motivate intelligence to solve complex problems (what good is feeling fear if you’re too dumb to figure out how to escape danger?) so the evolution of one reflects the evolution of the other.

Estimating chimp IQ from both brain size and reaction time

But even an IQ of -238 might overestimate chimp intelligence, because chimps not only have much smaller brains than humans, but reaction times more than 0.61 SD slower than the white mean.  Since brain size and reaction speed both correlate about 0.45 with IQ after correcting for range restriction and reliability, and research shows these variables correlate about 0.5 with one another, giving these variables standardized regression weights of 0.31 and 0.29 respectively, a multiple validity generalizations suggests chimps have IQs less than (-9.9 SD/0.31) – (0.61 SD/0.29) = -34.04 SD from the white mean.  In other words, an IQ less than -411.

Is an IQ of -411 plausible?

At first I thought this was nuts. For scholar Richard Lynn had estimated chimps have the mental ability of a white three year old. Surely having the mind of three year old puts your IQ well into the positive double digits.  For even the lowest deviation IQ in a billion whites is defined as IQ 10 (6 SD below the mean on a normalized curve) and far far more than one in a billion white adults has a mental age below three (those with profound retardation for example).

However in order for the IQ scale to be meaningful, it must be normed on only biologically normal people, and professional IQ tests exclude organic retardates and other impaired people from their samples, perhaps because those people have their own bell curves, and including them in the general population bell curve artificially inflates the left tail of the distribution.  When IQ norms are limited to the biologically normal, adults with toddler mentality become impossibly rare.

Further, three-year-old humans are much much much smarter than adult chimps are.  For starters, their brains are much closer in size to those of adult humans than they are to adult chimps’.

Secondly, unlike human three-year-olds, many chimps lack even the most basic self-awareness.  Commenter Santoculto has called self-awareness the mother ship of human intelligence noting that humans are the only animal that can think about our own thoughts and strategies.  This meta-cognition makes us light years more intelligent than chimps.

Thirdly, even one year old humans can ask questions, even just with one word (i.e. Mamma?).  Not even the brightest chimps that have been trained to answer many questions with sign language have the cognitive ability to ask them.  The ability to ask questions represents a quantum leap forward in intelligence, potentially equivalent to hundreds of IQ points.

It would explain a lot

The idea that chimps have an IQ as low as -411 may sound absurd, but it has some explanatory power.  Commenter Swank has claimed that chimps have an IQ of 40 (a figure I used to believe) to insightfully argue that racial intelligence differences must be very small since IQ 40 is not that much lower than human IQ.  If humans and chimps (separated for millions of years) only differ by a few dozen IQ points, then races separated by tens of thousands of years likely differ very little.

On the other hand, if the average British white has an genetic IQ 100, and the average chimps has an IQ less than -411, then that’s a gain of 511 IQ points in the 3.5 million years since apes left the trees, (more than 0.00015 points per year), predicting  a 15 point gain in the 100,000 years since modern humans left Africa (the same as the famous 15 point black-white IQ gap in the U.S.).

Secondly, if humans really are hundreds of IQ points smarter than chimps, it would explain why we haven’t been invaded by space aliens.  If humans are the much smarter than even our closest relatives, it implies human level intelligence is such an outlier, such a fluke, that nowhere else in the galaxy did that much brain power evolve.