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Introduction
It has long been known that head size and height are positively related to intelligence. This relationship was noted for head size in the nineteenth century by the French physician Paul Broca (1873), who measured the external and internal skull dimensions of brains at autopsy and observed that skilled workers had a larger average brain size than the unskilled, and eminent individuals averaged a larger brain than the less eminent. An early report of a positive association between head size and intelligence was published by Galton (1888) in a study of students at the University of Cambridge in which it was found that head size measured as circumference was greater by 2.5 to 5% in those who obtained top degrees, taken as a measure of higher intelligence, than in those who obtained less good degrees. Later in the nineteenth century, the relation between educational attainment (an approximate proxy for intelligence) and height was reported in a study of 33,500 American school students by Porter (1892).
Since these early reports, numerous studies have confirmed the positive relation between head size and intelligence. This positive association has been confirmed in numerous subsequent studies showing that head size is positively correlated with intelligence measured by intelligence tests, summarized by Lynn (1994) and Rushton (2000, p. 37), who reported results from 32 studies with an average correlation of 0.23. In another study, Rushton and Ankney (2009) summarized the results of 59 studies that reported the relation between external head measures and IQ for which the average correlation was 0.20. They also summarized the results of 28 studies that reported the relation between brain size measured by brain imaging and IQ, for which the average correlation was 0.40. The reason that the correlation was higher when brain imaging was used to measure brain size is that it is much more accurate than external head measures. In these studies head size was adopted as a proxy for brain size and it was argued that larger brains conferred greater cognitive ability. In more recent studies brain size has been measured directly by magnetic resonance imaging (rather than being inferred from head size) and was shown to be positively associated with intelligence with a correlation of 0.40 in a meta-analysis by Vernon