Is There Any Correlation Between Brain Size and Intelligence?

Correlation between brain size and intelligence
In a battle of wits, we find ourselves superior to animals. Also among humans, some demonstrate higher intelligence quotient test scores than others. It is said that intelligence is correlated with brain size. How far is this true?
Did You Know?
Our ancestors that include early primates, Hominids, Homo sapiens and finally the Neanderthals had a larger brain than what we have today.
Is a larger brain an indication of more intelligence? Is the difference in cognitive abilities between humans, and between humans and animals, associated with brain size? Does the brain size that can be measured by volume or weight have anything to do with mental processing? Let us find the answers.
Brain Size in Humans and Animals
It is a fact that humans are far superior to animals, when it comes to intelligence. Our cognitive competence is way ahead of animals. However, is our advanced cognitive ability related to brain size?
Human Bird Brain Size
When compared to the human brain, it is obvious that birds have smaller brains, whereas elephants and whales have larger brains. However, when body size is taken into consideration, it is observed that the human brain is 3 times larger relative to the body size. So if we take into account the brain/body size ratio we humans (1/40) are way ahead of elephants (1/560), lions (1/550), horses (1/600) and even sharks (1/2496).
Does that mean our brain size is the largest in proportion to body size, which is making us more intelligent than animals? Well, this reason falls flat when we compare brain-to-body ratio of humans with that of mice (1/40) and small birds (1/14).
Encephalization Quotient (EQ)
Considering that the brain-body size relationship has drawbacks, researchers have come up with another factor known as encephalization quotient (EQ) to measure intelligence. EQ is the ratio of the actual brain mass of an animal to the predicted brain mass of an animal of the same body weight. In such a case, the EQ score of humans is higher (7.5) than any other animal. The EQ score indicates that our brain is more than 7 times bigger than that of animals of the same body size.

However, the EQ measure assumes that larger the brain size, higher will be the neuron count. However, this assumption is incorrect since it does not consider neuron density, which if higher, reflects more neurons in the same area than others. To put it simply, if the neurons are tightly packed together, even a smaller brain can contain a greater number of neurons than a larger brain.
Neuron Density and Intelligence
Neuron Density and Intelligence
Studies suggest that intelligence probably depends upon the number of neurons lying in a specific part of the brain. The study was carried out by a team of researchers headed by Suzana Herculano-Houzel, an associate neuroscientist at the Federal University of Rio de Janeiro, Brazil.
In the study, it was observed that the brain of African elephants has around 250 billion neurons, which is almost thrice the number of neurons found in the human brain (86 billion). Now, the cerebral cortex (part of the brain) is involved in advanced-level functions such as cognition, language, sensory perception, decision-making, language, and memory. Researchers observed that the cerebral cortex of humans has three times as many neurons as observed in the cerebral cortex of African elephants. This higher neuron density in the cerebral cortex is probably giving humans an edge over African elephants in cognitive skills.

The study also observed that even the brain of small birds like parrots had densely packed neural cell bodies, making the total neural count more or less the same as observed in the brain of large animals such as apes. So it is the neuron density that might be playing a role in influencing intelligence.
Brain Size and Human Intelligence
Is a genius' brain larger in size as compared to the brain of an individual showing average intellectual capacity? The answer to this question can be best addressed by studying the brain structure of famous scientists like Albert Einstein. The scientist who is regarded as the 'embodiment of pure intellect', surprisingly had an average brain size. There was nothing extraordinary about his overall brain size.
Einstein on United States Postage Stamp
However, Einstein's brain showed a significantly larger prefrontal cortex, the part of the brain linked to execution of complex cognitive functions. Moreover, glial cells that ensure proper nutrient supply to the neurons, were also found to be significantly higher. His brain also showed unusually shaped inferior parietal lobes. All these neurological attributes (and not the overall brain size) might have been responsible for his extraordinary intellectual capacity.
Brain Size in Men and Women
After taking into account the body size, it is observed that brain size of men is larger than women. A study published in the journal Intelligence and conducted by a team of psychologists at the University of Western Ontario, Canada, observed that men tend to be more intelligent than women. In the study, scholastic aptitude test (SAT) scores of around 100,000 17- and 18-year old girls and boys, were taken into consideration. Each individual's SAT score was then used to calculate the general intelligence factor, commonly referred to as g factor. It was observed that the average IQ score of boys was greater than girls by 3.63 points.

However, Bruce Bracken, a psychologist at the College of William and Mary in Williamsburg, Virginia, argues that there were 10,000 more girls than boys in the study, which may have impacted the score. He suggests that boys who knew that they could not score good marks in SAT, did not even attempt the test. This factor has not been taken into consideration while evaluating the results of the study.
According to a 2005 study reported in the journal Intelligence, it has been concluded that people with a large-sized brain are more intelligent and have the ability to learn things faster than those with a smaller-sized brain. The study was conducted by Michael McDaniel, a psychologist at the Virginia Commonwealth University. The research has taken into account the results of over two dozen previous studies that used MRI scan to find a correlation between brain volume and intelligence.

The professor after extensively evaluating the results of the studies observed that as the brain volume increases, so does intelligence. In simple words, intelligence is directly proportional to the brain volume. In the study, individuals gave standardized individual tests and the results were compared to their brain volume.
According to McDaniel, these tests are an accurate way (critics don't agree) to evaluate an individual's capacity for reasoning, which can be the most crucial factor to assess his suitability for a designated job. McDaniel said that these intelligence tests are immensely helpful in predicting his performance at work.

The psychologist concluded that intelligence is related to brain volume and that it can be measured with these tests that reflect the individual's mental makeup. He said that evaluating job applicants by using these intelligence tests can be of use in hiring employees who will be faster at learning information, making significantly less number of errors and will be performance-oriented.
Critics argue that although the IQ score is helpful in giving a general idea about one's intelligence level, it certainly cannot define the cognitive processing ability of the person. This is because there have been many instances of people who had succeeded in professional life despite scoring low on IQ tests. On the other hand, some people with high IQ's were surprisingly found to be under performers.

Some researchers suggest that a greater brain size may indicate more brain tissue, which might result in improved intellectual capacity. On the whole, the brain has a very complicated structure, hence one singe factor such as its size cannot be the only predictor of intelligence. Other factors such as neuron density, the extent of neural connections between different parts of the brain and of course nutritional support for brain development, also affect the brain function and subsequently the cognitive abilities.