From time to time, people have reasoned that if there is a correlation between brain size and intelligence, then larger cranial size (including a large forehead) should be correlated with greater intelligence. In the early 1820s, Samuel George Morton, an American natural scientist and physician began collecting human skulls and by 1851 his collection exceeded one thousand trophies, including a large number from various indigenous tribes in the Americas. Assuming that cognitive ability is proportional to cranial volume, Morton filled each of the skulls with lead buckshot and measured its volume. Using this data, he concluded that whites are, on average, the most intelligent race. Aboriginals in America came second, and blacks came last. However, in computing the average skull size for aboriginals, Morton had included a large number of small-skulled Inca Peruvians and only a few of the larger skulled Iroquois. This gave a disproportionately small value for the aboriginal skull size, thereby invalidating his findings. Morton’s flawed conclusions were either a careless error in statistical inference or an early example of statistical skulduggery.
A more significant flaw in Morton’s approach was his tacit assumption that cognitive ability is proportional to cranial volume. If this assumption were true, we humans would have to explain why the cranial volume of Neanderthals was 1600 cc. compared with the modern human cranial volume of 1250–1400 cc. (Although recent studies do suggest that the Neanderthals may have been at least as intelligent as modern humans.) Morton’s study was also based on the assumption that cranial volume is indicative of brain size, yet this assumption is challenged by a 1994 study that asserted, “The correlation between external cranial size (head circumference) and brain volume is only about 0.288.” This indicates that there is a very weak correlation, between skull size and intelligence, but measures such as IQ are a better measure of our cerebral capabilities.
