Research from Washington University in St. Louis has identified variations in brain scans that they believe identify portions of the brain that are responsible for intelligence.

As suspected (and as explained by cartoons) brain size does play a small role; they said that brain size accounts for 6.7 percent of variance in intelligence. Recent research has placed the brain's prefrontal cortex, a region just behind the forehead, as providing for 5 percent of the variation in intelligence between people.

The research from Washington University targets the left prefrontal cortex, and the strength of neural connections that it has to the rest of the brain. They think that these differences account for 10 percent of differences in intelligence among people. The study is the first to connect those differences to intelligence in people.

Researchers took functional magnetic resonance imaging scans, or fMRIs, of participants while they rested passively. Their performance of tasks that tested their fluid intelligence (the ability to reason quickly and use abstract thinking) and cognitive control were conducted outside of the scanner, and researchers estimated on connectivity levels. The results of the tests were consistent with increased activity in the prefrontal cortex and higher levels of neural connectivity.

Study co-author Todd Braver, professor of psychology in Washington University's College of Arts and Sciences and of neurosciences at the university's School of Medicine, says that the research suggests that part of what it means to be intelligent is the hard-working nature of the left prefrontal cortex and the other part is the connections between that area and the rest of the brain.

The Washington University researchers believe the prefrontal cortex to be the "flexible hub" that uses its connectivity to monitor, coordinate, and guide other regions of the brain.

The study is perhaps limited by the fact that the tests were conducted outside of scanners, and thus some brain activity was estimated. In addition, some people, like psychologist Howard Gardner, believe in seven to nine other types of intelligences – verbal, visual, physical, musical, mathematical, introspective, interpersonal, naturalist, and existential.

The study was published in Neuroscience of Intelligence.