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Neurobiology of intelligence: science and ethics

Key Points

  • Intelligence research is more advanced and less controversial than is generally realized. Definitive conclusions about the neural and genetic bases of intelligence are being drawn — these have ethical implications that need to be addressed.

  • General intelligence and volume of frontal grey matter (assessed by magnetic resonance imaging) are correlated.

  • The lateral prefrontal cortex is consistently activated during intelligence testing. Frontal and parietal brain regions implicated in working memory are also activated under test conditions. These data contribute to the debate on whether intelligence has a unitary (activation of a single brain region/functional unit) or multiple basis.

  • The structure of brain regions that influence intelligence is under strong genetic control. Studies of intelligence using twins reared separately support the heritability of cognitive function.

  • Environmental factors — such as prenatal exposure to toxins, duration of breastfeeding and shared family environment — affect intellectual function. These non-genetic factors have a much greater effect on childhood intelligence in impoverished families.

  • Establishing a neurobiological basis for intelligence has important ethical implications. For example, is it ethical to assess racial differences in intelligence? Such questions need to be proactively addressed so that the field of intelligence research is not perceived as socially divisive.

Abstract

Human mental abilities, such as intelligence, are complex and profoundly important, both in a practical sense and for what they imply about the human condition. Understanding these abilities in mechanistic terms has the potential to facilitate their enhancement. There is strong evidence that the lateral prefrontal cortex, and possibly other areas, support intelligent behaviour. Variations in intelligence and brain structure are heritable, but are also influenced by factors such as education, family environment and environmental hazards. Cognitive, psychometric, genetic and neuroimaging studies are converging, and the emergence of mechanistic models of intelligence is inevitable. These exciting scientific advances encourage renewed responsiveness to the social and ethical implications of conducting such research.

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Figure 1: Studies of the biological bases of intelligence have identified relationships between variables at three broad levels of analysis: behaviour, biology and the wider context.
Figure 2: Frontal brain damage compromises fluid intelligence more than crystallized intelligence.
Figure 3: Linking genes, brain structure and intelligence.
Figure 4: Different methods of assessing the relationship between intelligence and the brain implicate similar brain regions.

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Acknowledgements

This work was supported in part by research grants from the National Institute of Mental Health to J.R.G., and from the National Institute for Biomedical Imaging and Bioengineering and the National Center for Research Resources to P.M.T. The authors thank R. J. Sternberg and W. R. Gray for their comments on a draft.

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DATABASE

Entrez Gene

ASPM

BDNF

CHRM2

COMT

CTSD

DRD4

IGF2R

MAOA

MSX1

FURTHER INFORMATION

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Thompson's homepage

Glossary

FLUID INTELLIGENCE

(Gf). 'On the spot' reasoning and novel problem-solving ability.

INTELLIGENCE QUOTIENT

(IQ). Intelligence scaled by age, standardized to have a population mean of 100 and SD of 15 (mental age divided by chronological age). It produces an intelligence ranking relative to other individuals of the same age, which tends to be stable across the lifespan of the individual.

CRYSTALLIZED INTELLIGENCE

(Gc). Performance guided by overlearned skills or knowledge, such as vocabulary.

g LOADING (OF A TASK)

A measure of how strongly individual differences in performance on a particular task predict differences in general intelligence (g). A task with a high g loading will better reveal differences between people of higher and lower intelligence.

WORKING MEMORY

A cognitive/neural system for maintaining information actively in mind (storage) and manipulating it (executive processing), or holding it in mind despite potential distraction or interference (control of attention).

PSYCHOMETRIC INTELLIGENCE

Intelligence as measured by an IQ-type test, typically assessing the accuracy of a response (and not the speed).

INSPECTION TIME

The minimum duration of exposure to a visual stimulus that a study participant requires to respond accurately about that stimulus.

GENETIC POLYMORPHISMS

Different variants of the same gene.

HOMEOBOX GENE

A key regulator of embryonic development, highly conserved across species, which controls cell and organ differentiation. Homeobox genes can activate many other genes, producing entire body segments.

QUANTITATIVE TRAIT LOCUS

(QTL). A genetic polymorphism that affects the expression of a continuously distributed phenotype. Typically, QTLs are statistically associated with trait variations that depend on multiple interacting loci.

POLYGENIC

A complex trait is polygenic if it is determined by multiple genes that each have small effects and that can interact with each other to produce effects.

ENDOPHENOTYPES

Gene products (phenotypes) that are not visible.

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Gray, J., Thompson, P. Neurobiology of intelligence: science and ethics. Nat Rev Neurosci 5, 471–482 (2004). https://doi.org/10.1038/nrn1405

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