I was recently asked about the relationship between brain size and intelligence. This is a question I get every now and then, because the relationship is not an obvious one. In this particular case, the questioner was curious about the fact that Neanderthals had a larger cranial capacity on average than modern Homo sapiens.

For the sake of this discussion I am going to avoid a detailed treatment of the concept of “intelligence.” It’s important to note that intelligence is a multifaceted phenomenon, there is no single way to measure it, and it cannot be captured by any single number. Having said that, we can use various markers or measures of cognitive ability to loosely represent overall intelligence. Problem solving, for example, is one measure, and can be used in multiple species. I will come back to this later, for for the sake of this discussion I will be referring to intelligence as overall cognitive ability without getting specific.

Once scientists figured out that the brain is the seat of intelligence, we were quick to observe that humans had relatively large brains. This made sense – humans seem to be, by far, the most intelligent species on the planet, so it makes sense that we have the biggest brains.

However, we don’t have the biggest brains. Whales and elephants, for example, have bigger brains. An average adult human brain weighs 1300-1400 grams. The brain of a killer whale weighs 5,620 g, a bottle nose dolphin brain weighs 1,500 – 1,600 g.  Elephants brain weigh over 5,000 grams while the sperm whale sports the heaviest brain at over 7,500 grams.

Weight is only one measure of “size,” however. We can also consider the number of neurons in the various brains. Humans have 86 billion neurons in their nervous system, and 19-23 billion neurons in the cerebral cortex. By comparison the African elephant has 11 billion neurons in their brain, while killer whales have 10.5 billion, and chimpanzees have 5.5–6.2 billion.

You may have noticed that the human brain has many more neurons than the elephant brain, despite the elephant brain being about 4 times heavier. This suggests that the human brain (actually the primate brain) is much more efficiently organized that non-primate brains.

Since it is clear humans don’t have the heaviest brains, it was proposed that brain size relative to body size is what matters. This scheme, however, fell short as well, as it favored small mammals, who should all be geniuses.

Finally scientists settled on the notion of encephalization quotient (EQ), which is brain size relative to body size compared to animals of your same phylum and size. Therefore small vertebrates would be compared to other small vertebrates.

The reason for this is that large bodies require larger brains (you need more neurons to handle the motor and sensory function of a larger body, for example). But not all parts of the brain scale together, and so the curve is different and different body sizes. EQ is a measure of how far above or below the body size to brain size curve a species or individual is.

Humans do have the highest EQ of any animal. However, the difference in number of neurons is only about 10% greater than a typical primate of our size. EQ does not seem to fully explain the intelligence advantage of humans. Some researchers feel it is the acquisition of specific cognitive skills, not just raw power, that sets humans apart. For example, humans have highly developed language skills, and something called a “theory of mind” which enables us to imagine what other people are thinking and feeling.

Conclusion

There are various ways to look at the relationship between brain size and intelligence between species. Weight relative to other vertebrates of a similar size (EQ) is one. Number of neurons relative to body size is another. However, you also have to consider the organization of the brain (cortical folding to increase surface area, for example) and the evolution of specific cognitive skills. Size matters, but it’s not everything.

Within species these factors all matter, but even more subtle properties also likely play a role. Within humans, for example, two people with the exact same brain size and body size might have very different levels of overall intelligence.  There are two other types of biological differences that may explain this. One is physiology and the other is the individual neuronal connections and circuits.

There are genetic variations in genes for proteins that affect neuronal function. Some individual may simply have neurons and nerves that function more efficiently – conduct signals more quickly, or have improved synaptic connections.

Further, the specific pattern in which neurons connect to each other must also matter. It is those connections that determine the information memory and processing in the brain. Some people, for example, are able to hold more bits of information in their working memory than others.

I said “biological” differences above because there are also environmental differences. Experience, education, culture, family environment, maturity, and training can all affect various aspects of intelligence.

So, how intelligent were our Neanderthal cousins? We can only infer from what they left behind, which seems to indicate that they were intelligent, but not as intelligent (despite their larger brains) as the Homo sapiens that eventually replaced them.