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If human brains could be made larger, how much smarter could we become? And is there a practical limit to brain size?

Marc Evans
Via email

Making the human brain bigger might sound like a straightforward way to boost intelligence, but it’s more complicated than that.

Intelligence isn’t just about size – it is also about things like how densely packed our neurons are and how well they are connected, as well as the brain’s structure and how efficiently it operates. There are also practical limits to increasing brain size.

Bigger brains require more energy, can cause complications during childbirth, generate more heat and may slow down signal transmission because of longer distances. Evolution has fine-tuned our brain size to balance cognitive abilities with these biological constraints.

So, if we are looking to enhance intelligence, it’s more effective to improve how our brains function, make use of technology and create better environments, rather than just trying to increase brain size.

As shown by Phineas Gage, having a tamping iron through the frontal lobes of your neocortex may not kill you

James V. Stone
University of Sheffield, UK

First, does brain size matter? There is a weak correlation between human brain size and IQ, so probably. A more compelling correlation can be found by looking across mammalian species, where there is an unambiguous correlation between brain size and general cognitive ability. For example, chimpanzees cannot play checkers, which is a game that children can play.

Second, if the human brain were made larger, the consequences would depend on which parts of the brain increased in size. A part of the brain called the cerebellum constitutes around 60 per cent of the brain in the mormyrid electric fish, which relies on the structure for its impressive electric-field sensing skills.

In contrast, human competence depends on a relatively recent increase in the size of the outer layer of the brain, called the neocortex. And it matters which part of the neocortex is allowed to expand.

As tragically demonstrated by Phineas Gage (1823–1860), having a tamping iron thrust through the frontal lobes of your neocortex may not kill you, and it may not affect your intelligence, but it will destroy your ability to plan ahead or to control impulsive behaviours. So, frontal cortex matters.

Third, not all neocortical neurons are born equal. Neurons from different mammal species vary in terms of size, the speed at which they conduct electrical signals, metabolic demands, the number of connections and plasticity (ability to make new connections).

Human neurons usually have a distinct advantage for each of these variables. Anecdotal observations agree with this overall impression. After clusters of neurons were cultured in a dish and then trained to play a game of Pong, human neocortical neurons performed substantially better than those from a mouse.

So, assuming any increase in human brain size was actually an increase in neocortex, a bigger brain should provide a corresponding increase in ability. The difference between a normal human and a big-brained human would probably be akin to the difference between a child’s view of the world compared with that of an adult. For example, a child sees the same images of the world, but fails to infer many of the hidden nuances implicit in those images.

We all know people who are clearly much smarter than ourselves; irrespective of the size of their brains, those people probably have all the properties of big-brained humans. With a bigger brain, we would see the same world as we see now, but we would understand so much more about it at any given moment.

For an insightful novel on this topic, see Flowers for Algernon by Daniel Keyes.

Hillary Shaw
Newport, Shropshire, UK

Making the human brain larger might not be a smart thing to do.

Say you increased its dimensions by a factor of 2. The head would now weigh eight times as much, but what supports it is the horizontal cross-sectional area of the neck and rest of the body, so you would need the body to scale up by √8, or 2.83. Then the body would be 2.83³, or 22.7, times as massive.

The new larger organism would require about 22 times the amount of food we currently consume, maybe a little less as heat losses per kilogram of body weight are reduced. Earth would support far fewer such humans (reducing the social capital and wisdom we can collectively accumulate).

Moreover, at that size, we would probably be slow-moving, grazing herbivores, so most of our time would be spent simply searching for and consuming grazing material, which has few calories per kilogram. That would give us less time to learn new things. Large animals also have to devote more brain power to simple locomotion.

The reverse logic of this suggests that we could still be smart with smaller brains, but much smaller brains would probably lack the complexity, the number of synapse connections, to be that smart.

In other words, we have probably already hit the sweet spot where the brain is big and complex enough to make space probes, vaccines and neutron bombs, but not so big as to be unsupportable or to have its knowledge-generating functions impaired.

Greta Blake
Darlington, County Durham, UK

Yes, there is a practical limit to brain size, it’s called the birth canal.

Just ask your female relatives and friends.

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