http://rationalwiki.org/wiki/RacialismThe head bone's connected to the...bigotry bone?
Racialists were also known for having big skull fetishes. The pseudoscience of phrenology was incorporated into racialist theories during the 19th century. The variant of phrenology known as "craniology" or "craniometry" became popular during this period. Craniology attempted to measure intelligence using the shape and size of the skull as a proxy. Craniology was used as a justification for all sorts of bigotry. Brits, for example, declared the Irish to be inferior to Anglo-Saxons based on skull measurements. It could also be called a form of "scientific" sexism as it was used to "prove" the superiority of men.[12] Surgeon and anthropologist Paul Broca conducted some of the most famous large-scale studies attempting to demonstrate the intellectual superiority of whites and men.[13] Herbert Spencer was also known as a promoter of these ideas in his psychological writings.
Craniology is, of course, made of multiple layers of wrong. In theory, it is already two steps removed from what it's attempting to measure, i.e., skull size (cranial capacity, in technical parlance) is used to approximate brain size which is in turn used to approximate "intelligence." Cranial capacity can, in fact, be used to measure brain size. However, more advanced bigots have generally moved on to direct measurements of brain size (thanks in large part to modern brain scanning technology), which is where the weak link in the chain really is. The problem here is that the size of the brain by itself is not a measure of intelligence. If this idea were taken to its logical conclusion, the world would likely be ruled by elephants or sperm whales. To use an example much more comparable to humans in brain structure, Neanderthals' brains were on average a bit larger than ours.[14] So much for the Neanderthal master race. What is much more important than absolute brain size includes factors such as cortical folding, neuronal organization, dendritic and synaptic connections, etc.[15]
Furthermore, the size and shape of the skull is itself not set in stone. As was demonstrated a hundred years ago by Franz Boas, people who are raised with a higher quality of life tend to have slightly larger and differently-shaped skulls than those who grew up in poverty, with little food and limited access to health care.[16] It was this research, showing that upbringing plays as much a role in skull size and shape as genetics does, that helped to discredit the "science" back in the early 20th century.
For good measure, and to demonstrate how utterly wrong craniological ideas are, assume that there is a single part of the brain responsible for intelligence and that its size magically corresponds exactly to intelligence levels (or, in other words, set the game up in the bigots' favor as much as possible). Because we know about the neuroplasticity of the brain, this still does not prove any kind of biological or genetic determinism tied to the size of this part of the brain as we don't know the direction of causality, i.e. is the person smart because the hypothetical "intelligence cortex" happens to be large or is that part of the brain large because the person happens to have learned well?
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