The researchers reported their findings in an article published in the journal Psychological Science.
According to the researchers, numerous studies have identified an association between total brain volume and intelligence since the 19th century. However, due to their lack of systemic control for critical confounding factors, such as height and population structure, many experts have found these studies' claims to be controversial. The researchers also observed that the relationship between brain size and cognitive performance seemed to grow weaker as the sample population grew larger.
To test the relationship between brain size and cognitive performance with much greater reliability, the researchers decided to conduct a large-scale study with a sample size about 70 percent larger than the combined samples of all previous studies. They obtained data for their analysis from the UK Biobank, an international health resource that contains detailed information from 500,000 British people. This medical research project offers a wealth of information to scientists, ranging from diet and work history to health records, brain and abdominal scans and blood biochemistry.
For their study, the researchers focused on analyzing the results of brief tests a subset of participants in the UK Biobank took for logic, memory and reaction time. But because these tests did not account for acquired knowledge, the researchers noted that the results were a "relatively noisy" measure of general cognitive performance. Nevertheless, using a model that incorporated different variables, they were able to determine which ones were predictive of better brain performance and educational attainment.
After systematically controlling for sex, age, height, socioeconomic status and population structure, the researchers reported finding a robust association between total brain volume and fluid intelligence, which is consistent with previous reports. They also found that total brain volume and educational attainment are positively associated. The researchers noted that these relationships were mainly driven by gray matter rather than the size of white matter or fluid volume. (Related: Maintaining brain health: Keep your brain young with these supplements and lifestyle habits.)
"It’s a simplified analogy, but think of a computer,” said UPenn Assistant Professor Gideon Nave, the lead author of the study. "If you have more transistors, you can compute faster and transmit more information. It may be similar in the brain to some extent. If you have more neurons, this may allow you to have a better memory, or complete more tasks in parallel."
Despite their findings, however, Nave also emphasized that things are more complex in reality. External factors, such as the environment a person grows up in, may also influence his cognitive performance. This influence may be what's being reflected by the association between his brain size and his performance on cognitive tests. Nave also cautions that just because brain size is predictive of brain performance, it doesn't mean that a person's capabilities can be measured based solely on the size of his head.
"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 only about two percent of the variability in test performance," Nave explained. "For educational attainment the effect was even smaller: an additional 100cm3 cup full of brain would increase an average person’s years of schooling by less than 5 months."
Philipp D. Koellinger, the senior author of the study, echoes this sentiment, adding that factors other than brain size exert a larger impact on human cognition. On the other hand, he believes that the influence of total brain volume is significant enough to warrant being controlled for in future studies, if researchers aim to explore the relationship between fine-grained measures of brain anatomy and cognitive health.
Besides these associations, the researchers also found a substantial difference between males and females in terms of brain volume. The difference, however, doesn't translate into differences in cognitive performance, which further suggests that other, as yet unidentified, factors are at play.