Intelligence and the Brain

What IQ tests measure, the 'g' factor, how brain size and networks relate to intelligence, genes and environment, the Flynn effect — and the myth of Einstein's extraordinary brain.

The g factor

In 1904 the British psychologist Charles Spearman noticed that children's scores on very different tests — maths, vocabulary, pitch discrimination — were positively correlated. He proposed a general intelligence factor, "g", underlying them. Modern IQ tests combine many subtests (verbal reasoning, working memory, processing speed, spatial reasoning) and still find this general factor, alongside more specific abilities. IQ predicts school and job performance reasonably well, but it captures only part of what people mean by intelligence — not creativity, wisdom or social skill.

Brain size and structure

Across healthy adults, overall brain volume correlates modestly with IQ — studies using MRI typically find correlations of around 0.2 to 0.3, meaning brain size explains only a small portion of the differences. Cortical thickness, white matter integrity and network efficiency matter too.

Networks, not a spot

The parieto-frontal integration theory (P-FIT), proposed by Rex Jung and Richard Haier in 2007, argues that intelligence depends on how efficiently a network of frontal and parietal regions, connected by white matter, integrates information. Some studies find that more intelligent people show more efficient patterns of brain activity, though findings are mixed.

Genes and environment

Twin and adoption studies show intelligence is substantially heritable — estimates rise from around 20% in infancy to about 50% or more in adulthood, within the populations studied. But heritability isn't destiny: it depends on environment, and thousands of genetic variants each contribute tiny effects. Environment matters enormously: nutrition, schooling, lead exposure and poverty all affect cognitive development. The Flynn effect — the rise in average IQ scores across many countries during the 20th century, roughly 3 points per decade — was far too fast to be genetic.

Einstein's brain

After Albert Einstein died in 1955, his brain was removed and studied. Various researchers claimed unusual features, such as more glial cells in one region or an unusual parietal lobe. These studies were based on one brain compared with small control groups, and are not considered convincing evidence of what made him brilliant.