Does IQ Correspond to Brain Anatomy or Functioning?

Stephen Jay Gould's influential book The Mismeasure of Man contended that intelligence testing represented a history of researchers distorting data to validate their biased preconceptions. A central argument in Gould's work was that intelligence constitutes a reification—an abstract concept mistakenly treated as real. He maintained that g lacked any meaningful link to the brain's physical or functional characteristics. Although this position had some merit when Gould published his initial edition in 1981, subsequent neuroscience research has revealed substantial connections between g and brain structure and function.

Neuroscience's contributions to intelligence research emerged relatively recently for two key reasons. The first is straightforward: throughout most of the twentieth century, neuroscience tools were insufficiently precise for meaningful intelligence research. Early investigators relied on studying brain injury patients, conducting experiments during neurosurgery, or performing postmortem examinations. Progress accelerated dramatically with technologies allowing examination of living brains, including electroencephalography (EEG), computed tomography (CT), magnetic resonance imaging (MRI), and later positron emission tomography (PET) and functional MRI (fMRI).

The second reason involves neuroscientists' traditional focus on universal principles of brain function rather than individual differences—the foundation of intelligence research. This divide between researchers studying general principles versus individual variations has long existed in psychology. However, since the 1980s, some neuroscientists have bridged this gap, producing evidence in the twenty-first century supporting g's connections to brain structure and function.

Contrary to Gould's assertions, multiple brain characteristics correlate with IQ scores. Brain size correlates with intelligence at r = .20 to .40 when measured through brain-imaging techniques in living subjects. Superior measures of g yield stronger correlations with brain size, and whole-brain measurements correlate more strongly than specific regions, though frontal lobe size shows particularly strong correlations with IQ. Notably, Gould dismissed extensive research demonstrating this relationship despite being aware of these studies.

Beyond brain size, researchers have identified additional correlates since the 1990s. White matter volume, which connects brain regions, correlates with both problem-solving speed and IQ, indicating that brain connectivity significantly influences intelligence. Grey matter is equally important, as neuron loss through disease associates with IQ decreases. More intelligent individuals possess more densely packed neurons that are better organized, with fewer chaotic branches (neurites) than those in lower-IQ individuals.

These correlations definitively refute claims that g is merely a statistical artifact or social construct. The prevailing theory today is the Parieto-Frontal Integration Theory (P-FIT), which proposes that g emerges from both the functioning of specific brain regions—primarily in the frontal and parietal lobes—and their interconnection via the arcuate fasciculus. An alternative theory by Euler builds on predictive processing, suggesting intelligence reflects the brain's capacity to manage unexpected situations.

While research into g's neurological basis remains relatively early-stage, accumulated correlational evidence makes it indisputable that brain functioning and anatomy correlate with g. Most remarkably, intelligence tests weren't designed to correlate with brain characteristics—yet they do. This provides extraordinarily strong evidence that g is real and stems from the brain's biology.



From Chapter 3 of "In the Know: Debunking 35 Myths About Human Intelligence" by Dr. Russell Warne (2020)