Volume of Gray Matter in Certain Brain Regions Is Highly Heritable and Correlates with Intelligence

The volume of gray matter in certain brain regions has been found to be highly heritable, that is, increasingly similar in persons with increasing genetic similarity (Thompson et al., 2001). This study also showed that differences in the amount of gray matter in the frontal lobes correlates closely with differences in intelligence quotient (IQ). This research bridges among neuroscience, genetics, and cognition. It was carried out by a team of eight American and five Finnish neuroscientists and behavioral scientists.

The 40 normal, healthy subjects included ten monozygotic pairs (MZ or identical twins) and ten dizygotic same-sex twin pairs (DZ or fraternal twins); half of the pairs of twins were male and half female; subjects averaged 48 years in age. The twins were drawn from a national Finnish registry. The genetic similarity between members of a pair (zygosity) was determined by DNA testing. Three-dimensional maps of gray matter were made by magnetic resonance imaging (MRI) for all subjects. Each subject also took a neuropsychological test battery that assessed 17 different cognitive domains. The two twins in a pair were tested by different examiners.

Within MZ pairs, gray matter measures were almost identical through a broad anatomical band encompassing frontal, sensorimotor, and language regions, including Broca's and Wernicke's areas. Within DZ pairs, gray matter measures correlated significantly for sensorimotor and parietal but not frontal cortex. The measures for DZ pairs showed less similarity than for MZ pairs but greater similarity than for random pairs of subjects. This continuum of decreasing similarity from MZ, through DZ, to unrelated pairs is strong evidence for heritability of cortical structure. Brain images for MZ, DZ, and unrelated pairs, color-coded to show levels of correlation, are available at this website.

The quantity of frontal gray matter was found to be significantly correlated with a composite score derived from different cognitive tests, suggesting that frontal gray matter is closely related to general intelligence. In Box 1.2, we noted that overall brain size is correlated with intelligence; the present study suggests that the cortex of the frontal lobes is particularly related to intelligence. Total cortical mass was less closely related to intelligence than was frontal cortex. In a commentary on this study, Plomin and Kosslyn (2001) note that the correlation reported by Thompson et al. between frontal gray matter and intelligence is only about as large (.40) as other investigators have reported between total brain volume and intelligence. But Plomin and Kosslyn claim that Thompson et al. underestimate the true correlation by calculating the association independent of other brain regions, whereas gray matter volumes in different brain regions are likely to intercorrelate substantially.

Does the correlation between gray matter and intelligence show that a larger volume of gray matter causes higher intelligence? Not necessarily, for example, it might occur because twins were raised and continue to live in similar environments. On pp. 576-581 we note ways in which differential experience affects a variety of brain measures.

Because performance on linguistic tasks is highly heritable, the investigators asked whether the structure of the language areas of the cortex is also highly heritable and if so, whether the heritability is higher in the left hemisphere, which contains regions specialized for language function. They found that the structure of the language areas is indeed highly heritable, and more than in similar areas in the right hemisphere. No other brain region was found to show hemispheric asymmetry for heritability of cortical structure, but the investigators point out that a sample of only 40 subjects may be too few to make general comparisons of heritability among cortical regions. For this and other reasons, the investigators are now expanding their study, initially adding another set of 20 pairs of twins (P. Thompson, personal communication, November 28, 2001).

Recent research has shown that many cognitive skills are highly heritable, and so are some personality traits and mental illnesses. Thompson and colleagues note that brain regions that are most similar among family members may be especially vulnerable to diseases that run in families, including some forms of psychosis and dementia. The heritability of schizophrenia is discussed in Chapter 16, pp. 506-507, and is shown in Figure 16.1.

References:

Plomin, R. and Kosslyn, S.M. (2001). Genes, brain and cognition. Nature Neuroscience, 4(12), 1153-1154.

Thompson, P.M., Cannon, T.D., Narr, K. L., van Erp, T., et al. (2001). Genetic influences on brain structure. Nature Neuroscience, 4(12), 1253-1258