You bring up a controversial question!
The relationship between brain size and intelligence, both amongst humans and between different species, has never been particularly well-defined. 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. As nature would have it, both of these common assumptions are incorrect. Whales and elephants have much bigger brains than humans, and we have about the same brain-to-body mass ratio as mice. 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). 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.
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?
Some studies claim the answer is yes.
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. 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. 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.
To be honest, I find these correlation a bit unsettling. 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. 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. 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. Correlational studies have only established a weak to moderate linear relationship between brain size an intelligence, which is enough fuel to ensure that the brain size and intelligence hypothesis doesn’t burn out, but does little to explain the true basis of human cognitive capacity.
Luckily, there is much more to a brain when you look at it under a microscope, and most neuroscientists now believe that the complexity of cellular and molecular organization of neural connections, or synapses, is what truly determines a brain’s computational capacity. This view is supported by findings that intelligence is more correlated with frontal lobe volume and volume of gray matter, which is dense in neural cell bodies and synapses, than sheer brain size. Other research comparing proteins at synapses between different species suggests that what makes up synapses at the molecular level has had a huge impact on intelligence throughout evolutionary history. So, although having a big brain is somewhat predictive of having big smarts, intelligence probably depends much more on how efficiently different parts of your brain communicate with each other.