Furthermore, a closer look at the results of the gene-association study reveal that most of the relationship the authors found between HMGA2 gene variations and cranial size could be accounted for by the fact that the gene is also correlated with human height. Although IQ is historically the most widely used intelligence measure, by no means does it account for all aspects of human intelligence, nor is it an entirely consistent readout of cognitive ability between individuals. Clearly, there is more to intelligence than brain size, or classic geniuses like Albert Einstein, who had an average-sized brain, would have been out of luck! It is important think about how we should actually define intelligence, and to keep in mind that the studies cited above only show a correlation between brain size and a person’s score on an intelligence quotient test. To be honest, I find these correlation a bit unsettling. More recently, a genome-wide association study which included 20,000 human subjects was widely reported by the media to have discovered an “IQ gene.” According to their results, certain variations in the HMGA2 gene, which codes for a protein that helps regulate DNA transcription and cell growth, are correlated with increased intracranial volume as well as enhanced IQ.
Ten years ago, a meta-analysis that examined the results from 26 imaging studies concluded that the correlation between IQ and brain volume is consistently in the 0.3-0.4 range. The emergence of magnetic resonance imaging (MRI) has made it possible to compare brain sizes of living humans, and in the ongoing hunt for a physical metric of intelligence, several researchers eagerly sought to correlate MRI measures of brain volume with IQ. Okay, so despite the uncertain relationship between brain size and cognitive abilities between different species, can brain size predict anything about intelligence amongst humans? Does having a gigantic brain mean that you’re smarter, as cartoons like Pinky and the Brain and Jimmy Neutron Boy Genius would have us believe? By this metric, at least, humans come out on top, with an EQ of 7.5 far surpassing the dolphin’s 5.3 and the mouse’s measly 0.5. Since it would be against human nature to admit defeat, scientists have crafted a third measure of brain size called the encephalization quotient, which is the ratio of actual brain mass relative to the predicted brain mass for an animal’s size (based off the assumption that larger animals require slightly less brain matter relative to their size compared to very small animals). Whales and elephants have much bigger brains than humans, and we have about the same brain-to-body mass ratio as mice. As nature would have it, both of these common assumptions are incorrect. Humans like to believe that our exceptional cognitive abilities must indicate that we are the kings of the animal kingdom in terms of brain size, or at least that we have the largest brains relative to our body size.
The relationship between brain size and intelligence, both amongst humans and between different species, has never been particularly well-defined.
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