England's idiom "highbrow", derived from the physical description of the skull that can barely contain the brain within it, comes from a long-standing belief in the existence of a relationship between brain and intelligence.
More than 200 years ago scientists have been looking for such an association. They began to use rough measures, such as estimated skull volume or head volume, the investigation has become sophisticated in the last few decades when MRIs offered a very precise brain volume calculation.
However, the relationship remained loose and full, and many studies failed to take into account confusing variables, such as altitude and socioeconomic status. Published studies are also subject to "publishing bias", the tendency of publishing only significant findings.
A new study, the largest of its kind, was conducted by Gideon Ship of the Wharton School of Pennsylvania University and by Philipp Koellinger of Vrije Universiteit Amsterdam, clarifying the link. Using information on brain size derived from MRIs related to the results of cognitive effects testing and measures of educational achievement gained from more than 13,600 people, researchers have found that, as previous studies have shown, there is a positive relationship between brain volume and brain performance in cognitive testing. But that finding comes with important warnings.
"The effect is there," says Nave, assistant marketing professor at Wharton. "On average, people with bigger brains tend to work better on cognition tests than those with smaller brains, but the size is just a small part of the picture, explaining about 2 percent of the variability in the test. The achievement effect was even smaller: additional & # 39; cup & "This implies the fact that other factors, besides this one factor who took so much attention over the years, received 98 percent of the second variation in the performance of cognitive tests," says Dr. Koellinger, .
"However, the effect is strong enough that all future studies that will strive to clarify the relationship between fine grain size of brain and cognitive health anatomy should control the overall brain volume, so we see our study as a small but important contribution to better understanding the differences in cognitive health" .
Lieutenant and Koellinger's associates in a paper published in the journal Psychological science, included Joseph Kable, Baird's professor at Penn's Department of Psychology; Wi Hoon Jung, a former post-doctoral researcher at Kable's Laboratory; and Richard Karlsson Linnér, a post at Koellinger's Laboratory.
From the outset, researchers have been trying to reduce the effects of bias and confusing factors in their research. They registered the study beforehand, ie they published their methods and promised to publish in advance so they could not simply bury the results if the findings were insignificant. Their analyzes also systematically controlled sex, age, height, socioeconomic status and population structure, measured by participant genetics. Height is associated with higher better cognitive performance, for example, but also with larger brain size, so their research has been making zero on the contribution of brain size per se.
Earlier studies have consistently determined the correlation between brain size and cognitive effects, but the relationship seemed to be weaker, as the studies involved more participants, and Nave, Koellinger and colleagues wanted to question the size of the sample that rejected the previous efforts.
The research relied on the recently collected data set, UK Biobank, providing information from more than half a million people across the United Kingdom. Biobank includes the health and genetic information of participants, as well as scanning images of a subgroup of about 20,000 people, a number that grows for a month.
"It gives us something that has not existed before," says Koellinger. "This sample size is gigantic – 70 percent bigger than all the previous studies on this subject put together – and allows us to test the correlation between brain size and cognitive performance with greater reliability."
Measuring cognitive effects is a difficult task, and researchers point out that even the estimation used in this study has weaknesses. Participants took a short questionnaire that tests the logic and ability of reasoning, but not the acquired knowledge, giving a relatively "noisy" measure of general cognitive performance.
Using a model that embedded different variables, the team looked to predict better cognitive performance and educational achievement. Even controlling other factors, such as height, socioeconomic status and genetic origin, the total brain volume was positively correlated with both.
The finds are intuitive. "It's a simplified analogy, but I mean a computer," says Nave. "If you have multiple transistors, you can calculate faster and transmit more information, perhaps the same in the brain. If you have more neurons, it can give you more memory or more parallel tasks.
"However, things can be much more complex in reality, for example, consider the possibility that a larger brain, which is very inherited, is associated with a better parent, in which case the link between the larger brain and the test results can simply reflect the influence of parenting on the knowledge. be able to get to the bottom of this without more research. "
One of the important findings of the analysis is the differences between men and women. "Just like height, there is quite a significant difference between men and women in brain volume, but this is not translated into the difference in cognitive performance," says Nave.
A more significant view of the brain scan can explain this result. Other studies have reported that in women, the cerebral cortex, the outer layer of the front of the brain tends to be thicker than in men.
"This could explain the fact that, despite the average relatively small brain, there is no effective difference in cognitive performance between men and women," says Nave. "And of course, many other things could happen."
The authors emphasize that a comprehensive link between brain volume and "brain marrow" is weak; No one should measure the size of the job during the employment process, Ship jokes. Moreover, what is excluded from the analysis is how little of the brain volume it explains. Factors such as parental style, education, nutrition, stress and others are probably the main contributors not specifically tested in the study.
"Previous estimates of the size of brain and cognitive abilities were uncertain enough that the actual relationship could be practically very important, or, alternatively, not much different from zero," says Kable. "Our research enables the field to be much more confident in the magnitude of this effect and its relative importance moving forward."
In the course of the paper, researchers are planning to zoom in to determine whether certain regions of the brain or their association between them have an extraordinary role in contributing to cognition.
They also hope that a deeper understanding of the biological background of cognitive impact can help to illuminate environmental factors, some of which may be influenced by individual action or government policies.
"Suppose you have the necessary biology to become a fantastic golf or tennis player but you will never have the chance to play so you never realize your potential," says Nave.
Koellinger adds, "We hope that if we can understand the biological factors that are related to cognitive performance, we can identify the environmental circumstances in which people can best manifest their potentials and stay cognitive health."
The research was supported by ERC Consolidator Grant, Wharton Neuroscience Initiative, and Wharton's Dean Research Fund.