How does the size of a brain affect the level of intelligence?



How does the size of a brain affect the level of intelligence?

In: Biology

There appears to be a slight link [between brain size and intelligence]( There’s a correlation of around 0.3 to 0.4. What this really means is about 10-15% of the variation in brain size is linked with intelligence. It’s not a very strong link, but it seems to be there.

But, it’s not clear which causes which. Do people with bigger brains find it easier to learn? Or does studying hard help your brain to grow bigger? We don’t know!

What that does mean is the most (85-90%) of the variation in brain size has nothing to do with intelligence.

It doesn’t seem to affect it very much. Just compare things like crows and sheep. Crows are *way* smarter (in the limited ways intelligence can be measured), but have much smaller brains. We don’t really know how the brain creates intelligence, so we can’t offer any good explanations for how differences in brain anatomy alter intelligence. We know that the brain works as a series of messages communicated between brain cells, and so theoretically, the more brain cells the more potential cells and connections between cells. We also know that the cerebellum – the wrinkly bit of the brain – locates all its cell bodies and connections on the surface, so a larger brain theoretically provides a greater amount of surface to pack more cells into (it’s wrinkled because this also increases surface area).

However, it seems that a lot of brain size isn’t used to the theoretical fullest potential, since small brains like those of crows can display much greater measurable intelligence than many larger-brained animals. There is almost certainly elements of how the brain is put to use and how efficiently the brain is built impacting this too. Perhaps a larger brain is more likely to evolve than a more complex but same-size brain, so when greater intelligence is important, only animals where natural selection requires a small size will select increased complexity mutations, and animals where they can get big without much trouble instead select increased size mutations. Or maybe natural selection didn’t favour increased brain power in the case of large animals at all, and in these cases a larger brain was just a side-effect of mutations increasing overall size for other purposes?

As for brain build, it’s known that brains are subdivided into various parts that do different jobs. For example, there’s a structure in the brain called the medulla, which influences a bunch of unconscious reflexes like breathing and heart rate modulation. These processes are handled specifically in the medulla and nowhere else. There are all sorts of regions like this. Differences in the sizes and purposes of these regions may result in impacts on perceived intelligence – a small brain that dedicates a lot of space to say, puzzle-solving ability, may seem a lot more intelligent than a large brain that’s focused mostly on memory storage. For the record, humans dedicate a huge amount of brain space to language processing, to the point that we actually have worse short term memories than many other mammals, cos we’ve given up these abilities for the sake of even better talking good.