Katherine Unger Baillie, Medical Xpress, November 30, 2018
A new study, the largest of its kind, led by Gideon Nave of the University of Pennsylvania’s Wharton School and Philipp Koellinger of Vrije Universiteit Amsterdam, has clarified the connection. Using MRI-derived information about brain size in connection with cognitive performance test results and educational-attainment measures obtained from more than 13,600 people, the researchers found that, as previous studies have suggested, a positive relationship does exist between brain volume and performance on cognitive tests. But that finding comes with important caveats.
“The effect is there,” says Nave, an assistant professor of marketing at Wharton. “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 is only a small part of the picture, explaining about 2 percent of the variability in test performance. For educational attainment the effect was even smaller: an additional ‘cup’ (100 cubic centimeters) of brain would increase an average person’s years of schooling by less than five months.” Koellinger says “this implies that factors other than this one single factor that has received so much attention across the years account for 98 percent of the other 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.”
From the outset, the researchers sought to minimize the effects of bias and confounding factors in their research. They pre-registered the study, meaning they published their methods and committed to publishing ahead of time so they couldn’t simply bury the results if the findings appeared to be insignificant. Their analyses also systematically controlled for sex, age, height, socioeconomic status, and population structure, measured using the participant’s genetics. Height is correlated with higher better cognitive performance, for example, but also with bigger brain size, so their study attempted to zero in on the contribution of brain size by itself.
Earlier studies had consistently identified a correlation between brain size and cognitive performance, but the relationship seemed to grow weaker as studies included more participants, so Nave, Koellinger, and colleagues hoped to pursue the question with a sample size that dwarfed previous efforts.
“This gives us something that never existed before,” Koellinger says. “This sample size is gigantic — 70 percent larger than all prior studies on this subject put together — and allows us to test the correlation between brain size and cognitive performance with greater reliability.”
Using a model that incorporated a variety of variables, the team looked to see which were predictive of better cognitive performance and educational attainment. Even controlling for other factors, like height, socioeconomic status, and genetic ancestry, total brain volume was positively correlated with both.
The findings are somewhat intuitive. “It’s a simplified analogy, but think of a computer,” Nave says. “If you have more transistors, you can compute faster and transmit more information. It may be the same in the brain. If you have more neurons, this may allow you to have a better memory, or complete more tasks in parallel.
“However, things could be much more complex in reality. For example, consider the possibility that a bigger brain, which is highly heritable, is associated with being a better parent. In this case, the association between a bigger brain and test performance may simply reflect the influence of parenting on cognition. We won’t be able to get to the bottom of this without more research.”
One of the notable findings of the analysis related to differences between male and females. “Just like with height, there is a pretty substantial difference between males and females in brain volume, but this doesn’t translate into a difference in cognitive performance,” Nave says.
A more nuanced look at the brain scans may explain this result. Other studies have reported that in females, the cerebral cortex, the outer layer of the front part of the brain, tends to be thicker than in males.
“This might account for the fact that, despite having relatively smaller brains on average, there is no effective difference in cognitive performance between males and females,” Nave says. “And of course, many other things could be going on.”
[Editor’s Note: The complete study, “Are Bigger Brains Smarter? Evidence From a Large-Scale Preregistered Study,” is available here.]