More than 100 years of empirical research provide conclusive evidence that a general factor of intelligence (also known as g, general cognitive ability, mental ability and IQ (intelligence quotient)) exists, despite some claims to the contrary. Intelligence can be reliably measured, is stable in rank-order across the lifespan, and is predictive of many important life outcomes, including educational and occupational success, health and longevity.
Intelligence shows high heritability in quantitative genetic studies; this heritability increases across the lifespan to mid-adulthood and partly overlaps with genetic variance that influences brain structure.
As with many other highly heritable complex traits, the genetic polymorphisms underlying normal-range intelligence differences remain elusive. One possible explanation is that many mildly harmful, lineage-specific, rare genetic variants ('mutation load') might be responsible for the heritability of intelligence.
The most robust finding in the neuroscience of intelligence is that larger brains, and a greater volume of grey matter in various regions in the brain, are associated with higher intelligence.
Intelligence does not reside in a single localized area in the brain. The available evidence suggests a widely distributed network of parieto-frontal brain areas underlies intelligence.
The distributed nature of intelligence in the brain suggests a crucial role of white matter integrity and an efficient neurological network structure. Both hypotheses have initial empirical support.
Functional efficiency (that is, low energy consumption in task-relevant brain areas) is also related to higher intelligence, especially when task difficulty is neither particularly high nor particularly low.
Various lines of evidence suggest that men and women might use their brains differently to achieve similar levels of cognitive performance. These sex differences might extend to individual differences: people might differ in how they use their brains to solve the same cognitive tasks.
Neuroscience is contributing to an understanding of the biological bases of human intelligence differences. This work is principally being conducted along two empirical fronts: genetics — quantitative and molecular — and brain imaging. Quantitative genetic studies have established that there are additive genetic contributions to different aspects of cognitive ability — especially general intelligence — and how they change through the lifespan. Molecular genetic studies have yet to identify reliably reproducible contributions from individual genes. Structural and functional brain-imaging studies have identified differences in brain pathways, especially parieto-frontal pathways, that contribute to intelligence differences. There is also evidence that brain efficiency correlates positively with intelligence.
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The work was undertaken by The University of Edinburgh Centre for Cognitive Ageing and Cognitive Epidemiology, part of the cross-council Lifelong Health and Wellbeing Initiative. Funding from the Biotechnology and Biological Sciences Research Council, Engineering and Physical Sciences Research Council, Economic and Social Research Council and Medical Research Council is gratefully acknowledged.
The authors declare no competing financial interests.
- Raven's Progressive Matrices test
An established non-verbal test of inductive reasoning that is often regarded as a good marker of the general factor of intelligence.
- Non-verbal reasoning
A broad subfactor of intelligence defined by tests that do not rely on verbal stimuli or responses. The term perceptual–organizational ability is often used synonymously.
A quantifiable phenotype with an assumed intermediate role in the pathway from genes to complex phenotypes. It is thought that the action of the endophenotype is easier to understand biologically and genetically than the action of the complex phenotype of primary interest.
- Mutation–selection balance
An evolutionary genetic explanation for the maintenance of genetic variance in a trait, based on an equilibrium between novel detrimental mutations and purifying selection.
- Small-world network
A network characterized by high levels of local clustering among nodes and short paths that globally link all nodes, resulting in all nodes being linked through few intermediate steps despite few connections per node.
- Long association fibre
A member of a set of axonal tracks connecting distant brain areas in the same hemisphere.
- Network efficiency
Describes short mean path lengths for parallel information transfer — as provided by a small-world network structure, for example.
- Functional connectivity
Correlations between the activation patterns of different brain areas.
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Deary, I., Penke, L. & Johnson, W. The neuroscience of human intelligence differences. Nat Rev Neurosci 11, 201–211 (2010). https://doi.org/10.1038/nrn2793
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