Size isn’t everything – it’s what you do with it that matters

Written by Jenny Straiton

Are bigger brains better? Researchers find that having a bigger brain might make you smarter, but only a little bit.

Human brain size

In the largest study of its kind, using data from over 13,600 people, researchers from the University of Pennsylvania (PA, USA) have found that there is a weak positive correlation between brain size and intelligence. After accounting for and controlling various confounding factors such as age, sex, height, socioeconomic status and genetics, results from the study suggest that brain size does affect intelligence, though not by much.

“The effect is there,” commented lead author Gideon Nave. “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 square centimeters) of brain would increase an average person’s years of schooling by less than five months.”

Collaborator Philipp Koellinger from Vrije Universiteit Amsterdam (Netherlands) added, “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.”

Previous studies have suggested the correlation, though research has been subject to publication bias with only those with positive results being published. They have also often failed to account for the confounding variables. Sex is known to correlate to brain size, as is height, though neither translates to differences in cognitive performance. This may be explained in that, despite a smaller overall volume, the female brain often has a thicker cortex.

“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” explained Nave. “And of course, many other things could be going on.”

Past studies have also used a much smaller data set, with the relationship seeming to grow weaker as the sample group grew. In the present study, data was taken from the UK Biobank and included the health and genetic information of patients alongside MRI scans of their brain. “This sample size is gigantic, 70% 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,” commented Koellinger.

It must be noted that the questionnaire-based evaluation methods used in this study have their weaknesses and the correlation shown is only weak, therefore, brain size should not be used as a stand-in for measuring intelligence. The researchers state that what really should be taken away from the results, recently published in Psychological Science, is how little the effect brain volume actually has.

Building on the results of this study, the next step for the researchers is to look at specific regions of the brain, or connections between them, and gain a deeper understanding of the biological basis of cognitive performance.

“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 surface of the iceberg here,” concluded Koellinger.