Genetic continuum of similarity in brain structure. Differences in the quantity of gray matter at each region of cortex were computed for identical and fraternal twins, averaged and compared with the average differences that would be found between pairs of randomly selected, unrelated individuals (blue, left). Color-coded maps show the percentage reduction in intra-pair variance for each cortical region. Fraternal twins exhibit only 30% of the normal inter-subject differences (red, middle), and these affinities are largely restricted to perisylvian language and spatial association cortices. Genetically identical twins display only 10-30% of normal differences (red and pink) in a large anatomical band spanning frontal (F), sensorimotor (S/M) and Wernicke's (W) language cortices, suggesting strong genetic control of brain structure in these regions, but not others (blue; the significance of these effects is shown on the same color scale).

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