The relationship of head size and intelligence quotient (IQ) to neuroanatomic similarities in young, healthy adult monozygotic (MZ) twins was examined using MRI and quantitative image analysis at the Department of Neurobiology, Harvard Medical School, Boston, MA. Significant effects of genotype but not birth order were found for the following measures: forebrain volume, cortical surface area, callosal area, head circumference, and full-scale IQ Forebrain volume, cortical surface area, callosal area, and each brain measure and head circumference were correlated, but not IQ. 
COMMENT. Brain measures in monozygotic twins are correlated with head circumference and brain volume but not with intelligence quotients. Intellectual similarities in MZ twins are not related to genetic influences on brain size (“bigger is ‘not’ better”) but rather, brain organization. Intellectual functions are correlated with regional brain measures and local geometry of folds in the left cerebral cortex and forebrain, areas that govern language and abstract reasoning.