The English idiom "highbrow," derived from a physical description of a skull barely able to contain the brain inside of it, comes from a long-held belief in the existence of a link between brain size and intelligence. For more than 200 years, scientists have looked for such an association. Begun using rough measures, such as estimated skull volume or head circumference, the investigation became more sophisticated in the last few decades when MRIs offered a highly accurate accounting of brain volume.Yet the connection has remained hazy and fraught, with many studies failing to account for confounding variables, such as height...