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In some cartoons, the one with a bigger brain is often described as "smart," but is it true in real life?

Hosted by: Brit Garner
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Sources:
http://discovermagazine.com/2010/sep/25-modern-humans-smart-why-brain-shrinking
http://www.larspenke.eu/pdfs/Pietschnig_et_al_2015_-_Meta-analysis_brain_volume_IQ.pdf
https://www.nature.com/articles/s41467-018-04268-8
http://journals.sagepub.com/doi/full/10.1177/2058460117703816
https://www.cell.com/current-biology/abstract/S0960-9822(16)30197-X
https://www.ncbi.nlm.nih.gov/pubmed/17655784?dopt=Abstract

Image Sources:
https://commons.wikimedia.org/wiki/File:Homo_habilis-cropped.jpg
https://commons.wikimedia.org/wiki/File:Neuron_Hand-tuned.svg
https://en.wikipedia.org/wiki/File:Gyrus_sulcus.png
[ ♪INTRO ].

If you’ve ever seen the cartoon Pinky and the Brain, you already know that bigger brains are way better. And if only those scientists had made that mouse’s brain just a little bigger, maybe he could have actually taken over the world. *sigh*.

But, it turns out the cartoon lied to us. Shocking, I know. In real life, things are much more complicated, and size isn’t everything — or even much, really, when it comes to intelligence.

It might be tempting to think that brain size is really important because it’s one of the things that makes us stand out. Relative to our bodies, our brains are bigger than the brains of other primates. And they’ve been getting bigger over time — our ancient relative Homo habilis had a brain one-third the size of ours, for example.

So you’d think that among people, those of us with the biggest brains would also be the smartest. But… while it’s true our species has evolved larger brains over the past two million years or so… that growth stopped around when we developed stone tools. And since then, our brains have actually been shrinking.

In the last 20,000 years, our brains have shrunk by 10% — that’s almost the size of a baseball, which is a lot of brain to lose. And... I mean… if size was everything, that would mean we’re a lot dumber now.

We’re not, though… Right? And that actually makes sense when you look at much more recent research on brains and intelligence. In the past half-century or so, lots of psychologists and neuroscientists have tried to draw connections between brain size — usually volume or weight — and intelligence, as measured by IQ tests or other standardized exams.

And they just haven’t found a solid relationship. For example, a 2015 meta-analysis combined the results of 88 studies on over 8,000 brains, and found that size only accounted for a little less than 6% of the variation in IQ between people. They also found that earlier studies published higher correlation between brain size and IQ.

Which either means that the connection between intellect and size has been disappearing over time… or that earlier studies were biased against publishing negative results. Regardless, scientists just can’t seem to agree that size really matters. And slowly but surely, other factors have been emerging.

New research suggests size is less important than connectedness, for example. And not how you might expect, because having highly connected neurons doesn’t make you smarter. Instead, less is more.

This connectedness can be measured by looking at what scientists call arborization in the brain. That’s the number and shape of dendrites – the long spindly branches neurons use to connect to other neurons — which scientists can estimate using neurite orientation dispersion and density imaging, cutely abbreviated to NODDI. It might seem pretty natural to assume that people with tons and tons of dendrites would have an advantage — more neuronal connections, more computational power, right?

But… when scientists examined the brains of 259 participants in a 2017 study, they found the opposite. There was a weak but significant negative correlation between the number of dendrites and test scores. And that suggests efficiency is better than quantity — which isn’t as surprising as you might think, if you think about it this way:.

Imagine your friend just moved and you’re trying to find your way to their new place. If there are like a million roads you could take to get there, it’ll take you forever to find the right one, and you’ll waste time and effort on wrong turns. But if there’s just one direct route right to their door, you’d be there in a jiffy.

The same seems to hold true for neurons. The fewer dendrite branches there are, the easier a time the brain has firing the right sequence of neurons. And ultimately, that means quicker, more efficient thinking.

But when it comes to wiring, simplicity isn’t the only key. To stick with that house-finding analogy: it’s easier to get to a place that happens to be in your neighborhood instead of two towns over. There’s just less of a chance you’ll get lost if the journey is short.

In your brain, those neighborhoods are created by wrinkles on the surface, which are known as gyri and sulci — gyri being the mounded bits, and sulci being the gaps. Those wrinkles are there so we can fit more brain inside our skulls, kind of like how crunching up a piece of paper allows it to fit into a smaller container. And conveniently, these folds let neurons with similar functions group closer together.

Instead of having to stretch connections all the way across a flat surface, neurons can more easily talk with neighbors that are squashed up in the same or a nearby gyrus. And there is evidence that variations in the shape of sulci and gyri are associated with general cognitive ability in humans. A 2016 study published in Current Biology looked at the brains and cognitive abilities of 440 adults and 662 children.

High resolution structural imaging was used to calculate the local gyrification index, or LGI — a measurement of the extremeness of brain folding. When compared to participants’ performances on tests designed to probe cognitive ability, the researchers found that in both adults and children, more extreme levels of folding were associated with better scores. In fact, the structure of the folds predicted about 12% of the variance in cognitive abilities in one of their samples.

The researchers responsible for the study suggested that this may be largely because of folding in areas of the brain that are multimodal — ones where a lot of functions are performed in a small area. More folding could make those areas better able to communicate and process information by putting important neurons closer together. But... even when we take into account brain wrinkles and things like dendrite arborization, there’s still a lot of variation in intelligence that isn’t well explained.

One thing is for sure, though — size is only a tiny piece of the puzzle. Luckily, there are plenty of other aspects of our brains that scientists can investigate to figure out what makes us so clever. Thanks for watching this episode of SciShow Psych!

If you want to learn more about your brain and how it works, stick around! This channel is all about exploring the science of the mind. And you can get new episodes sent straight to your YouTube subscription feed by clicking that subscribe button! [♪ OUTRO ].