There is a commonly expressed view that mental abilities are linked to brain size in humans. Such a view is partially supported by gradual increase of brain size in the course of human evolution. But does this mean that people with larger brains are smarter?

To answer this question, we have to provide a clear way of measuring “smartness”. This is not as simple as it sounds since individual mental abilities are hard to measure with a single test. Some people excel in math, others in languages, arts, technology, communication and so on.

For instance, there is hardly a clear way of directly comparing the intelligence levels of a painter and a brain researcher. A painter might be brilliant at producing highly valuable objects of art but most likely will know close to nothing about brain research. The same could be said about the abilities of a brain researcher in visual arts. More importantly, analytical and associative skills required for science and arts appear to be controlled by different parts of brain.

In the absence of a single and obvious way of measuring intelligence, some simplified tests like IQ (Intelligence Quotient) are generally accepted as a relatively good working tool. IQ measurements are not without problems, and there is a lot of criticism about their relevance. However, IQ test draws the final score from several types of mental abilities (verbal, logical, mathematical, and analytical) and, importantly, stood the test of time in the research field of measuring intelligence.

IQ testing utilizes standardized tests such as the WAIS–III (Wechsler Adult Intelligence Scale, Third Edition). The average IQ is between 90 and 110. The scores below the average may indicate varying degrees of pathology, such as borderline intellectual function, or mental retardation of various severity. Above the average range is typically linked with intelligence, brightness, giftedness, or genius.

The assumption that the size of the brain should correlate with the level of intelligence appeals to common sense. After all, our brain consists of interconnected neurons which are responsible for performing all brain functions. A bigger brain should be able to accommodate more neurons and thus have bigger capacity for storing and handling information.

But like with computers of several generations, size and capabilities do not exactly correlate. Modern laptops weighing about 1-2 kilograms can store more information and perform more tasks than supercomputers from the 1980s that used to occupy large buildings. This is a difference between quantity and quality. It is reasonable to assume that certain neural networks might be better at handling and processing information than others. Such networks might be relatively small compared to the brain size, in which case the absolute size of brain becomes not particularly relevant.

Recently, a comprehensive meta-analysis was conducted to answer the question about any connection between brain size and intelligence in humans. When the numbers were crunched for the results of 88 studies, using 148 research samples involving over 8,000 men, women, and children, the conclusion was that brain size does indeed correlate with the IQ level. The correlation, however, was rather weak. Importantly, the authors of the article noticed a publication bias whereby the positive and strong correlations between the IQ and brain size were published, while the data with less convincing numbers were ignored. Taking this into account, the authors of analysis conclude that the brain size is not necessarily the reason for the differences in the IQ of modern humans.

This is all interesting, but then there is a good evolutionary argument. There are no doubts that the size of the human brain has increased tremendously in the last couple of million years, from around 600 cm³ in Homo habilis to an average of 1200 cm³ in Homo sapiens. The rise of our intelligence was most certainly linked with this increase of brain size.

And here we come again to the question of quantity and quality. It is well known that the people with the largest muscles are not necessarily the strongest – there are different muscle fibers with different capacity of producing force. If the muscle fibers are the same, however, a bigger person will be stronger. The same can be told about brain and its neuronal networks. People with the same brain size can have neurons or certain brain regions working with different efficiency, thus leading to marked differences in the IQ levels.

It is of great interest that the brain size of Neanderthals (1600 cm³) was much larger than in modern humans. We don’t know how Neanderthals would have performed in the IQ test, but we know that they were out-competed by Homo sapiens, even though Neanderthals were physically superior to our ancestors. It is quite likely that our higher level of intelligence played some role in this event.

Many researchers now held the view that it is not the entire brain which is larger in a person with a higher IQ, but rather certain areas that are denser and may be larger. An interesting recent discovery is that these areas can be increased in size and improved in functionality during our life, thanks to the process called neuroplasticity.

Neuroplasticity refers to the dynamic process of maintenance and repair that our brains are constantly undergoing. We are born with about 100 billion neurons, but lose about 200,000 a day to a natural process called pruning. As we grow and develop, pathways in the brain which are unneeded are disconnected, and the neurons die. However, in a compensatory process, new neurons are produced and more elaborate connections are established between neurons which are more active. This is the process of constant de-cluttering, in which our brains grow to operate at maximum efficiency.

A famous and well publicized study of cab drivers in London determined that a specific bilateral structure in their brains had grown larger and denser as they navigated complex London streets, presumably without GPS. This structure is the hippocampus, which are located in the right and left temporal lobes of the brain, behind our temples. This is proliferation at work.

The hippocampus is responsible for storing and revoking the long-term memories, including the ability to recall visual images, which is essential for spatial–temporal orientation. In response to the increased work load of carrying out complex orientation and navigation tasks, the number of connections in the hippocampi in the cab drivers increased, making the hippocampi denser, and as an individual component of the brain, larger. There was a similar study showing that some parts of brain increase in size in response to musical training.

To conclude, we can say that yes, size matters, but the quality is definitely more important than quantity. In addition, the quality can be improved with training. IQ is not something fixed for life, it can be improved similar to many other characteristics of our body.

References

Kolb, B., & Whishaw, I. (1998). BRAIN PLASTICITY AND BEHAVIOR Annual Review of Psychology, 49 (1), 43-64 DOI: 10.1146/annurev.psych.49.1.43

Maguire EA, Woollett K, & Spiers HJ (2006). London taxi drivers and bus drivers: a structural MRI and neuropsychological analysis. Hippocampus, 16 (12), 1091-101 PMID: 17024677

Pietschnig, J., Penke, L., Wicherts, J., Zeiler, M., & Voracek, M. (2015). Meta-analysis of associations between human brain volume and intelligence differences: How strong are they and what do they mean? Neuroscience & Biobehavioral Reviews, 57, 411-432 DOI: 10.1016/j.neubiorev.2015.09.017

Silva, M.A. (2008). Development of the WAIS-III: A Brief Overview, History, and Description. Graduate Journal of Counseling Psychology.1, Article 11.

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