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Brain size, head size, and intelligence quotient in monozygotic twins

From the Department of Neurobiology, Harvard Medical School (Dr. Tramo), Boston, and the Department of Neurology, Cognitive/Behavioral Neurology Unit(Dr. Tramo), Massachusetts General Hospital, Boston, MA; the Program in Cognitive Neuroscience (Dr. Gazzaniga), Dartmouth College, Lebanon, NH; the Departments of Community and Family Medicine, Biostatistics and Epidemiology Group (Dr. Stukel), Psychiatry (Dr. Green), and Radiology (Dr. Weaver), Dartmouth Medical School, Lebanon, NH; and the Center for Neuroscience (W.C. Loftus), University of California, Davis, CA.

Address correspondence and reprint requests to Dr. Mark Jude Tramo, Dept. of Neurobiology, Harvard Medical School, 220 Longwood Avenue, Boston, MA 02115.

Many studies of monozygotic (MZ) twins have revealed evidence of genetic influences on intellectual functions and their derangement in certain neurologic and psychiatric diseases afflicting the forebrain. Relatively little is known about genetic influences on the size and shape of the human forebrain and its gross morphologic subdivisions. Using MRI and quantitative image analysis techniques, we examined neuroanatomic similarities in MZ twins and their relationship to head size and intelligence quotient (IQ). ANOVA were carried out using each measure as the dependent variable and genotype, birth order, and sex, separately, as between-subject factors. Pairwise correlations between measures were also computed. We found significant effects of genotype but not birth order for the following neuroanatomic measures: forebrain volume (raw, p 0.0001; normalized by body weight, p = 0.0003); cortical surface area (raw, p = 0.002; normalized, p = 0.001); and callosal area (raw, p 0.0001; normalized by forebrain volume, p = 0.02). We also found significant effects of genotype but not birth order for head circumference (raw, p = 0.0002; normalized, p 0.0001) and full-scale IQ (p = 0.001). There were no significant sex effects except for raw head circumference (p = 0.03). Significant correlations were observed among forebrain volume, cortical surface area, and callosal area and between each brain measure and head circumference. There was no significant correlation between IQ and any brain measure or head circumference. These results indicate that: 1) forebrain volume, cortical surface area, and callosal area are similar in MZ twins; and 2) these brain measures are tightly correlated with one another and with head circumference but not with IQ in young, healthy adults.

Supported by NIH awards DC00071 (to M.J.T.) and NS17778 (to M.S.G. and M.J.T.) and Office of Naval Research award N00014-89-J-3035 (to M.S.G. and M.J.T.).

Received August 29, 1996. Accepted in final form December 3, 1997.

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