What makes some people smarter than others? Is intelligence innate? Is it under genetic control? Is there something different about the brains of people with high versus low intelligence, and if so, what is the nature of the difference? Some answers to these important questions on this often touchy subject are emerging.
Many would bristle at the very notion that some people are “smarter” than others, if that is meant to imply an innate difference in ability. There is however a wealth of evidence that that is precisely the case, though it is important to define exactly what is meant by intelligence. When people are examined on a variety of tests, spanning different cognitive abilities – verbal ability, spatial reasoning, abstract logic, memory – it is found that people who do well on one of these tests tend to also do well on the others. Psychologists use the term “g”, for general intelligence, to denote a statistical construct which captures this correlation and which is thought to reflect some underlying characteristic that contributes to success on all these measures.
Results from a large number of twin, family and adoption studies agree that the heritability of g is very high – at least 50% and perhaps as high as 80%. (This means that 50-80% of the variance in g across the population is due to differences in genes). Some effects of a shared family environment are seen at early ages but these tend to disappear when examined in older individuals. Whatever the effect of the family environment on IQ measures when an individual is within that environment, these effects seem to be temporary and diminish in later life.
(An important aside: Note that the heritability within populations does not tell us anything about what might cause a difference in the mean of a trait between two populations. This is a common misinterpretation – differences in mean between populations may be entirely due to differences in environment, even if the trait is very highly heritable within each population, where environmental variance is low).
Presumably, some parameter of brain structure or function that correlates with intelligence is being affected by these genetic differences. While a correlation with overall brain size has been repeatedly noted (“Check out the big brain on Brett!”), this leaves a lot of the variance in intelligence unexplained (and is also not particularly informative). Is intelligence localized to a certain brain region or is it a distributed property of the entire network?
A number of recent studies, taking very different approaches, arrive at the same conclusion – it is the connectivity between areas of the brain that best correlates with intelligence. Paul Thompson and colleagues analysed brain connectivity and intelligence in a large twin study (with 92 pairs of twins). Using diffusion tensor imaging, they were able to assess the size, organization and “integrity” of axonal tracts connecting all areas of the brain. They found, when comparing these measures across pairs of either monozygotic or dizygotic twins, that these parameters were more heritable for some tracts than for others. Most importantly, they also found that intelligence was correlated with connectivity measures across numerous tracts in the brain. They could also show a substantial shared genetic effect – the genes affecting structural connectivity were also affecting intelligence.
Ralph Adolphs and colleagues used a very different approach – they analysed a large collection of patients with lesions in different parts of the brain. By looking across this collection for sites where lesions were consistently correlated with reduced intelligence they were able to map a network of important regions. The most striking finding is that many of the “regions” thus defined were actually within the white matter – they were not restricted to specific cortical areas but rather reflected the connections between areas of the brain. Again, the implication is that it is the efficiency and effectiveness of brain connectivity which are the major parameters affecting intelligence.
While the overall genetic effects are very robust, only a few specific genes have been identified that seem to influence intelligence. For now, the neurodevelopmental processes which mediate the effects of these and other genes on the parameters of brain connectivity remain a mystery.
Chiang, M., Barysheva, M., Shattuck, D., Lee, A., Madsen, S., Avedissian, C., Klunder, A., Toga, A., McMahon, K., de Zubicaray, G., Wright, M., Srivastava, A., Balov, N., & Thompson, P. (2009). Genetics of Brain Fiber Architecture and Intellectual Performance Journal of Neuroscience, 29 (7), 2212-2224 DOI: 10.1523/JNEUROSCI.4184-08.2009
Glascher, J., Rudrauf, D., Colom, R., Paul, L., Tranel, D., Damasio, H., & Adolphs, R. (2010). Distributed neural system for general intelligence revealed by lesion mapping Proceedings of the National Academy of Sciences, 107 (10), 4705-4709 DOI: 10.1073/pnas.0910397107