What brain size does - and doesn’t - say about smarts

A study led by VU professor of Genoecnomics Philipp Koellinger and Gideon Nave of the Wharton School found that people with larger brains rated higher on measures of cognitive performance and educational attainment. Size was far from everything, however, only explaining about two percent of the variation in smarts.

12/10/2018 | 4:12 PM

For more than 200 years, scientists have looked for a link between brain size and cognitive performance. Yet the connection has remained hazy and fraught, with many studies failing to account for confounding variables, such as height and socioeconomic status. Koellinger and Nave have clarified the connection, with the largest study of its kind. Using MRI-derived information about brain size in connection with cognitive performance test results and a measure of educational attainment, obtained from more than 13,600 individuals, they found that there is a positive relationship between brain volume and performance on cognitive tests, but that finding comes with important caveats.

Brain size: a small but important factor

On average, a person with a larger brain will tend to perform better on tests of cognition than one with a smaller brain. But size explains only about two percent of the variability in test performance. Koellinger: “This implies that factors other than this one single variable that has received so much attention over the years account for 98% of the variation in cognitive test performance. Yet, the effect is strong enough that all future studies that will try to unravel the relationships between more fine-grained measures of brain anatomy and cognitive health should control for total brain volume. Thus, we see our study as a small, but important, contribution to better understanding differences in cognitive health among people.”

Male versus female

One of the notable findings is related to differences between male and females. “There is a pretty substantial difference between males and females in brain volume, but this doesn’t translate into differences in cognitive performance,” Nave says. 

A more nuanced look at the brain scans may explain this result. Other studies reported that in females, the cerebral cortex—the outer layer of the front part of the brain—tends to be thicker than in males. 

Biology or opportunity?

The authors underscore that the overarching correlation between brain volume and “braininess” was a weak one; no one should be measuring job candidates’ head sizes during the hiring process. Indeed, what stands out from the analysis is how little total brain volume seems to explain. Factors such as parenting style, education, nutrition, personality traits, and others, are likely major contributors that were not specifically tested in the study.

In follow-up work, the researchers plan to zoom in to determine whether certain regions of the brain, or connectivity between them, play an outsized role in contributing to braininess. They’re also hopeful that a deeper understanding of the biological underpinnings of cognitive performance can help shine a light on environmental factors that contribute, some of which can be influenced by our actions and our governments’ policies. 

“Suppose you have the necessary biology to become a fantastic golf or tennis player, but you never have the opportunity to play, so you’ll never realize your potential,” Nave says. And Koellinger adds, “We’re hopeful that if we can understand the biological factors that are linked to cognitive performance, it will allow us to identify the environmental circumstances under which people can best manifest their potential and remain cognitively healthy. We’ve just started to scratch the top of the iceberg here.”