Not only the volume and thickness of gray matter in the brain (neuron cell bodies) but also the integrity and function of white matter (myelinated axons) in the temporal, frontal, and parietal cortices are related to intelligence.

Intelligence has a significant genetic component, with a percentage of genes producing functional proteins implicated in various neuronal functions, including synaptic function and plasticity, cellular interactions, and energy metabolism.

There is gene expression associated with the main neurons of the cortex and mesencephalon—pyramidal and medium spiny neurons. Studies suggest a possible relationship between the function and structure of pyramidal cells and human intelligence, specifically regarding dendritic size, action potential speed, and IQ. The integrity of multiple white matter tracts in the right uncinate fasciculus, connecting parts of the temporal lobe with frontal lobe areas, is crucial in defining intelligence. Damage in this pathway has been observed in individuals with intellectual disabilities, while good connectivity is seen in individuals with high IQ. This corresponds with gray matter volume in the frontal and temporal lobes.

Functional and structural neuroimaging studies show that general intelligence cannot be attributed to a specific region. Intelligence involves a network of regions such as the dorsolateral prefrontal cortex, parietal lobe, and anterior cingulate cortex, multiple regions within the temporal and occipital lobes, and major white matter tracts.

Frontal and parietal areas are related to fluid intelligence, the ability to think and reason abstractly, and solve problems. Temporal lobes, on the other hand, are associated with crystallized intelligence, which involves knowledge gained from prior learning and past experiences, as well as the integrity of white matter affecting processing speed, which defines gray matter volume.

As I define in the Intelligence DWRI, the left side of the prefrontal cortex, related to logical reasoning independent of acquired knowledge and correlated with IQ tests, is a precursor to overall cognitive development. Intelligence is also the result of the integrity of neuronal participation, synapses, and genetic composition.

The prefrontal cortex serves as a data distribution hub, monitoring and influencing other brain regions. Its ability to solve logic problems in unusual situations, regardless of acquired knowledge, designates this region as the intellectual leader and manager of all intelligence.

Neurons in individuals with higher IQs can translate input and output in action potentials between neurons more effectively and achieve greater synaptic integration resolution in pyramidal cells than those with lower IQs. Genes support dendritic structure in cognitive capacity, and dendritic size is related to intelligence.

Today, we have better results and discoveries about how intelligence functions, using neuroimaging, GWAS genetic studies, cellular neuroscience in human brain tissue, among others, which reveal how human intelligence works, with individuals evaluated in IQ tests as a reference.

To make an analogy: the genotype for intelligence is the region of logical reasoning, while the phenotype for intelligence is cognition.

Until five years ago, no genes had been associated with IQ test performance. Today, predictions are becoming more accurate, but further study is still needed to understand the impact of environmental factors on the development of intelligence.

The idea that DNA influences IQ, health, and behavior is known as “genetic determinism.” According to Harvard geneticist David Reich, the evidence is strengthening in favor of a genetic basis for intelligence. He questions, “Is performance on intelligence tests or the number of years of education someone receives shaped by upbringing? Certainly. However, does it measure something related to some aspect of behavior or cognition? Almost certainly. Given that all traits influenced by genetics are expected to vary between populations due to different frequencies of genetic variations, genetic influences on behavior and cognition will also vary between populations.

Fabiano de Abreu Agrela Rodrigues, columnist