In a news release by Elaine Schmidt titled "International team uncovers new genes that shape brain size, intelligence" published online at the UCLA newsroom, April 15 2012:
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DNA comprises four bases: A (adenine), C (cytosine), T (thymine) and G (guanine). People whose HMGA2 gene held a letter "C" instead of a "T" at a specific location on the gene possessed larger brains and scored more highly on standardized IQ tests.
However brain size is different than observable skull size. Early scientific attempts at studying skull size as a proxy for brain size and seeing whether that correlated with intelligence have since been debunked.

Respecting the broad diversity among human beings, caution must be used to avoid stereotyping people by their physical characteristics.