"g" has a number of correlates in the brain. Studies using magnetic resonance imaging (MRI) have established that "g" and total brain volume are moderately correlated (r~.3–.4). External head size has a correlation of ~.2 with "g". MRI research on brain regions indicates that the volumes of frontal, parietal and temporal cortices, and the hippocampus are also correlated with "g", generally at .25 or more, while the correlations, averaged over many studies, with overall grey matter and overall white matter have been found to be .31 and .27, respectively. Some but not all studies have also found positive correlations between "g" and cortical thickness. However, the underlying reasons for these associations between the quantity of brain tissue and differences in cognitive abilities remain largely unknown.