A Bigger Brain is Not Necessarily Better

Key Points:

• Having an unusually large brain doesn't necessarily make someone a genius, and large-scale research suggests only a slight and tenuous relationship between brain size and intelligence.
• Across vertebrate species, it's possible that it's the size of the brain relative to the body—rather than the brain's absolute size—that confers the greatest level of intelligence.
• But neuroscientists still aren't sure what humans' extra brain mass accomplishes—especially considering that much of our brains are inactive for much of the time.

People assume that having a big brain means that you’re especially intelligent. And if you are smart, you must have a big brain. But there is little evidence for either notion.

Consider an objectively smart person like Albert Einstein. There has been a decades-long quest to find the potential uniqueness of Einstein’s brain, but it is most notable for the bitter disputes over custody of various parts of his brain (the story is colorfully recounted in the book Driving Mr. Albert by Michael Paterniti). There is no evidence that his brain was special in any way.

Does a Big Brain Make You Smart?

Being “smart” doesn’t mean you have an especially big brain. The inverse is also not true: having a big brain doesn’t necessarily make you smart.

Take Edward Rulloff, an upstate New Yorker of the 19th century, who by some accounts had the biggest brain on record. His preserved brain is today on public view in the Wilder Brain Collection at Cornell University in Ithaca, New York. Rulloff was certainly well known in his time and until recently had a (quite mediocre) bar in Ithaca named for him. But a better term for Rulloff would be notorious: he murdered his wife and daughter, as well as at least one and possibly three other men. He was the last person to be hanged in New York. Though regarded by some in his time as a clever outlaw, there is just as much justification for crediting Rulloff’s big brain for his sociopathy as for his supposed intelligence. Which is to say no justification.

Why do we continue to associate big brains with smarts? Many studies over the years have systematically investigated associations between the size of the whole brain and performance on intelligence and language tasks. The largest and most recent meta-analysis (a study of studies) on the topic was led by Martin Voracek of the University of Vienna in 2015. Voracek and colleagues found that, although there is evidence for a relationship between overall brain volume and various intelligence measures, the strength of the relationship is very weak.

Encompassing 148 samples totaling more than 8,000 individuals in total, the Voracek study found that about 6 percent of the variance in intelligence performance can be attributed to differences in brain volume. This means that a very “intelligent” person (by one metric or another) can attribute only a tiny fraction of their smarts to having a big brain, while a highly “unintelligent” person can assign their brain size the same very small degree of blame.

However, even these weak relationships may be illusory. Showing that there is a positive relationship between brain size and smarts is an inherently headline-grabbing finding. Researchers who believe in such a relationship are more likely to undertake these kinds of studies, more likely to analyze their data in a way that generates the desired outcome, and more likely to be able to publish their positive results. Biases like these, which have riled the psychological sciences in recent years, make me doubt that any meaningful relationship exists between brain size and intelligence. Moreover, the original motivation for this whole line of investigation into brain size and intelligence has hideously racist roots (for a historical deep dive of this area, see this book).

While relationships between brain size and intelligence have been extensively studied, collectively pointing to very weak—and possibly illusory—relationships, other putative traits have been investigated much less. One can concoct just as plausible neuroscientific explanations why empathy, sociability, creativity, emotional granularity, and lots of other traits should be related to brain size, either big or small. The point is not that these other traits are more likely to be related to brain size; the point is that brain size tells us little, if anything.

Examining Brain Size in the Animal Kingdom

We can gain some perspective on brain size and intelligence if we look at other species. Animal brains vary greatly in size. Fruit flies have a brain about the size of a poppy seed. We have brains about the same volume as a medium-size pitcher (48 oz.). And a sperm whale’s brain is equivalent to about two gallons. Does this mean whales are much smarter than us? No—and in fact, animals with much smaller brains can show behavior that seems just as sophisticated as that of a whale.

Source: Elianne Dipp/Pexels

Whales may well be “smarter” than fruit flies but consider a slightly larger flying animal in comparison to the whale: a hummingbird. Comparative neurologist Barbara Finlay (who curates the Wilder Brain Collection) and colleagues have pointed out that hummingbirds and whales show patterns of behavior that are quite similar in sophistication. In fact, the tiny hummingbird may have the edge:

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Both sing (the hummingbird adds a courtship dance), defend territories and mates, raise young, and migrate seasonally for long distances. The hummingbird also builds nests and solves some interesting pattern-recognition problems in finding flowers… There is really no justifiable metric of behavioral complexity that would account for most of the excess poundage of the whale brain.

This example highlights the fact that brain size needs to be considered as a proportional measure. You also need to consider a species’ overall size. And it turns out that both hummingbirds and whales have brains that are about the size you would expect given how big their bodies are.

Why Are Humans' Brains So Big?

What about us? Humans have a particularly large brain for an animal our size. We are a “scaled-up” primate. So probably some of our achievements as a species are down to having more brains than would otherwise be expected for a creature with our body size.

But it is not the case that this excess is caused by one especially big part of our brain, such as an abnormally big prefrontal cortex to do more complex cognitive tasks. Instead, most parts of our brain are a bit bigger than they should be. In other words, our extra smarts are not associated with one or two especially big bumps on our brain.

Yet even our somewhat too-big brains are a puzzle. Neuroscientists have struggled to explain what our extra brain mass actually accomplishes. The best guess seems to be that, at the species level, our extra brain mass allows us to store more lifetime memories. One piece of evidence for this is that bigger-brained (and therefore bigger-bodied) mammals also tend to live longer than smaller mammals. In this view, bigger brains would be needed to keep track of personal memories from more days of being alive. But the jury is still out on this.

In any case, it is not true that more brain mass means a better memory at the individual level. People with “highly superior autobiographical memory,” who can remember events decades in the past with almost cinematic detail, show no systematic enlargement in brain areas related to memory such as the hippocampus. In fact, in some cases, these individuals have smaller brain volumes in relevant areas.

Ultimately, having a bigger brain probably wouldn’t help you. As I described in a previous post, your metabolism sharply limits how your brain operates. Only about 10 percent of your neurons can be active at a time. The brain is so costly in terms of energy consumption that we may be better off using even less of it at a given moment.

This principle may apply to brain mass as well. As an individual, having a somewhat smaller brain may make it easier to maintain the brain as a whole—especially one that, in our species, is already "scaled up." With metabolism employed more efficiently in a smaller brain, you might just be able to think better or remember more with less brain mass.

There isn't evidence yet that “too much” brain matter in an individual is less efficient. But there is certainly no reason for anyone to suffer brain-size envy.

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