A larger brain size does not guarantee a higher intelligence quotient (IQ), according to a new study that contradicts the belief that a bigger brain means an increased IQ potential.
With the advent of brain imaging methods (eg MRI, PET), reliable assessments of in-vivo brain volume and investigations of its association with IQ are now possible, researchers said.
An international team of researchers carried a meta-analysis examining correlations between in-vivo brain volume and IQ.
Based on the data from 148 samples comprising over 8000 participants, they report a robust but weak association between brain size and IQ.
This association appeared to be independent of participant sex and age.
"The presently observed association means that brain volume plays only a minor role in explaining IQ test performance in humans. Although a certain association is observable, brain volume appears to be of only little practical relevance," said Jakob Pietschnig from the Institute of Applied Psychology of the University of Vienna.
"Rather, brain structure and integrity appear to be more important as a biological foundation of IQ, whilst brain size works as one of many compensatory mechanisms of cognitive functions," said Pietschnig.
The importance of brain structure compared to brain volume becomes already evident when comparing different species, researchers said.
When considering absolute brain size, the sperm whale weighs in with the largest central nervous system. When controlling for body mass, the shrew is on the top of the list.
Similar results emerge when considering other aspects of species anatomy: Homo sapiens never appears at the top at the list, as would be expected.
Rather, differences in brain structure appear to be mainly responsible for between-species differences in cognitive performance.
Within Homo sapiens, there are indications that render a large association between IQ and brain volume similarly questionable.
For instance, differences in brain size between men and women are well-established, yielding larger brains of men compared to women.
However, there are no differences in global IQ test performance between men and women. Another example are individuals with megalencephaly syndrome (enlarged brain volume) who typically show lower IQ test performance than the average population.
"Therefore, structural aspects appear to be more important for cognitive performance within humans as well," said Pietschnig.
The study was published in the journal Neuroscience and Biobehavioral Reviews.
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