Researchers have long suspected that brain size affects intelligence, with people who have bigger brains tending to have higher IQs.
A new set of studies provides compelling evidence that this relationship is likely causal and not just correlational.
Key Takeaways:
- Among siblings, the one with the larger brain size tends to have the higher IQ. This suggests the correlation is not just due to family environment.
- Genetic factors influencing brain size also influence IQ, ruling out family environment as the sole explanation.
- Statistical analyses indicate the genetic overlap is consistent with brain size causally affecting IQ, not the other way around.
- The number of neurons, rather than just brain size, may be the key factor driving cognitive ability differences.
Source: Intelligence
Siblings Analysis Shows Brain Size-IQ Correlation Persists Within Families
Researchers have long known that there is a moderate correlation, typically around 0.3, between overall brain size and scores on IQ tests.
But it has been unclear whether this reflects a true causal relationship or whether it is driven by confounding factors.
For example, family environment could potentially influence both brain development and cognitive ability in ways that account for the correlation.
To address this, researchers identified families with multiple siblings and compared brain size and IQ within each family.
If the sibling with the bigger brain tends to have the higher IQ, it suggests the correlation cannot be entirely due to family environment.
The analysis included over 1,300 families with siblings, including both twins and non-twins.
Brain size was measured either directly with MRI scans or indirectly using head circumference.
The key finding was that even within families, the sibling with the larger brain size tended to have the higher IQ.
This was true for both MRI-measured brain volume and head circumference.
The results were similar when looking at just twins or just non-twins.
The researchers concluded that family environment cannot fully explain the observed correlation between brain size and IQ.
Genetics Studies Confirm Brain Size-IQ Link Not Just Environmental
Twin studies have previously suggested genetics influences both brain size and IQ.
But it was still possible that family environment could be driving the overlap between these traits.
To address this, the researchers looked at genetic data from large genome-wide association studies (GWAS) of both brain size and IQ proxies like years of education.
GWAS identify DNA variants associated with particular traits across the population.
The genetic correlation estimated from the GWAS data was significant, around 0.4.
This indicates that many of the same genetic factors influence both brain size and IQ.
Environmental confounding cannot fully explain the overlap.
The researchers also used a novel statistical method called Latent Causal Variable modeling to determine whether genetics are consistent with brain size causally affecting IQ.
The results indicated that about 70% of genetic influences on brain size are propagated to IQ.
Far fewer genetic factors affected IQ independently of brain size.
This asymmetry matches the prediction if brain size is causally influencing IQ.
The researchers concluded that genetics do not support family environment as the sole driver and instead point to brain size as causally upstream of IQ.
Neuron Numbers May Be Key to Cognitive Ability
What aspect of bigger brain size causes higher IQ? The number of neurons may be more important than overall volume.
Larger animals like whales and elephants have much bigger brains by weight than humans.
But humans have the most neurons, which are cells responsible for information processing.
Primates have relatively constant neuron size across a wide range of brain sizes.
So increases in brain volume likely reflect increasing neuron numbers.
And neuron numbers may be what truly drive cognitive capabilities.
The genetic evidence also points to neurogenesis, the creation of new neurons, as being important for IQ.
So within humans, differences in neuron numbers resulting from brain growth may underlie cognitive ability differences.
Overall brain size seems to be a useful proxy for this.
Implications and Future Research
The accumulating evidence continues to suggest that brain size has a causal influence on intelligence within humans.
This lends support to the theory that the remarkable expansion of human brain size over evolutionary time has been driven by selection for higher general cognitive ability.
There are still limitations though and room for further research.
More precise and reliable measurements of IQ factors could reveal even stronger brain size effects.
Studies parsing genetic influences on different brain regions could also shed light on which areas are most critical.
And larger neuroimaging genetics studies are needed.
While not definitive, the new findings help strengthen the case that within our species, having a bigger brain contributes to being a smarter human.
This basic relationship likely underlies the tremendous expansion of human intellectual prowess made possible by our bulbous brains.
MRI and Genetics Both Point to Brain Size Impacting IQ
The correlation between overall brain size and IQ has long been suspected but never conclusively proven.
Now two new sets of studies provide compelling evidence that the association is likely causal.
Researchers first looked within families and found the sibling with a bigger brain tends to have a higher IQ.
This held up whether brain size was directly measured with MRI scans or approximated by head circumference.
Since siblings share family environment, this suggests the correlation is not merely environmental.
The researchers then analyzed genetic data from large genome-wide studies.
The genetic overlap between brain size and IQ proxies was significant after accounting for potential environmental confounding.
Statistical modeling further indicated the genetic pattern is consistent with brain size causally affecting IQ, not vice versa.
Together these studies demonstrate that familial environment cannot fully explain the observed link between brain size and cognitive ability.
The evidence instead points to a direct causal relationship between the two.
Density & Number of Neurons Impact Intelligence
Larger animals like whales and elephants have much bigger brains by weight than humans.
This led to the mystery of why humans are so much more intelligent despite having smaller brains on average.
The answer is likely that brain size comparisons should focus on numbers of neurons rather than just overall mass or volume.
And humans have many more neurons than other species.
Within humans, increases in brain size probably reflect greater numbers of neurons.
And more neurons translate to better cognitive performance.
So for individuals of our species, having a larger brain pays big dividends for your smarts.
This illustrates how brain size can have different implications between versus within species.
For evolution, the key was expanding neuron counts.
For individual humans today, a bigger brain still contributes to higher IQ.
New Statistical Methods Allow Stronger Inferences
Past studies were limited in the inferences they could make about brain size affecting IQ.
Confounding factors like family environment could not be ruled out.
But new statistical genetic methods allow much stronger causal conclusions.
By analyzing genetic correlations, researchers can determine if the link between brain size and IQ is partly genetic.
This rules out family environment as the sole explanation.
Even more powerfully, methods like Latent Causal Variable modeling can use genetic data to infer the likely direction of causation.
The asymmetry of genetic effects in this case pointed to brain size as causal for IQ.
Together with the persistent sibling correlation, the genetic analyses make a compelling case that brain size has a direct causal influence on intelligence.
The combination of study designs allowed researchers to disentangle correlation from causation.
The Evolutionary Drive to Boost Brainpower
Scientists have long pondered the tremendous expansion of human brain size over our evolutionary history.
The new findings lend credence to the theory that this was driven by selection for higher general cognitive ability.
Making brains bigger requires large energy costs.
The benefits would have to be significant to sustain such pressures.
And intelligence provides one of the clearest advantages of having a bulging brain.
Of course, the human story is more complex.
Factors like the social environment likely also selected for brain expansion.
But the core causal link between brain size and IQ seems central to the extraordinary neural investments that led to human brilliance.
Our outsized brains have granted us exceptional smarts.
The new studies help confirm that within humans, having more gray matter continues to translate to greater mental power.
It provides micro-level evidence consistent with the broad trajectory of human cognitive evolution.
Limitations and Future Research Directions: Brain Size & IQ
While compelling, the new findings come with important limitations that suggest directions for future research.
The reliability of IQ measurements could be improved.
More precise quantification of specific cognitive abilities that relate to brain size will help further characterize the causal relationships.
Larger neuroimaging genetics studies are needed to identify associations between distinct brain regions and narrower dimensions of intelligence.
This can shed light on which areas are most critical.
More work is also needed on potential pleiotropic effects where genes influence brain size and cognition through independent pathways.
The analyses so far suggest these effects are limited, but further studies can help rule out this possibility more definitively.
And as always, more data will help strengthen the inferences.
Larger samples could provide greater resolution on the within-family patterns and asymmetry of genetic associations.
While not yet definitive, the accumulating research continues to point towards a causal impact of brain size itself on human intelligence differences.
Ongoing studies along these lines will further elucidate this fundamental relationship.
Takeaways for Cognitive and Brain Research
The recent findings on brain size and IQ have several implications for researchers:
- They demonstrate the value of complementary study designs including within-family, twin, and molecular genetic analyses to make inferences about causality. Combining multiple methods allows more robust conclusions.
- They illustrate how newer statistical genetic techniques like LD Score regression and Latent Causal Variable modeling can make stronger inferences from GWAS data than past methods. These will likely become important tools for future research.
- They reinforce the need for reliable and specific cognitive measurements in genetics studies. Psychometric considerations like internal consistency can affect the apparent relationships observed.
- They highlight the importance of neuronal counts over simple volume for understanding the effects of brain size differences. Neuron numbers may be critical for elucidating causal pathways.
- They provide molecular genetic leads regarding the role of neurogenesis and neuronal regulation pathways in cognitive abilities. Integrating genetic findings with brain imaging remains a priority.
- They lend credence to the theory that cognitive evolution has been an important selective force acting on the remarkable expansion of human brain size over time. This helps advance our understanding of human origins.
Overall the findings showcase how multiple research strategies can be combined to make headway on longstanding scientific questions like the brain size-IQ relationship.
Ongoing efforts promise to further unravel these fundamental connections between our biology and our mentality.
References
- Study: The causal influence of brain size on human intelligence: evidence from within-family phenotypic associations and GWAS modeling
- Authors: James J. Lee et al. (2019)
