In a three-part series at Psychology Today, Hobart and William Smith College computational neuroscientist Daniel Graham, author of An Internet in Your Head: A New Paradigm for How the Brain Works(2021), tackles that question:

First, most parts of the human brain are already larger than they should be for an animal life form of our size. But the difference is hardly commensurate with average human intelligence vs. average chimpanzee intelligence. Sure enough:

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.

(March 9, 2021)

Daniel Graham, “A bigger brain is not necessarily better” at Psychology Today

Meanwhile, there’s also the fact that many humans get by — in human society — with a split brain or only half a brain. As Graham notes, Albert Einstein’s brain, as such, was not unusual.

Second, the human brain has been shrinking:

In the last 30,000 years, our behavior has become dramatically more complex, leading to the birth of civilization. This period saw major advances in tools, weapons, architecture, and art. As Henneberg has found, based on measurements of fossil skull size, the brain of our species appears to have decreased in size by around 10 percent over this period. Roughly, this works out to as many as five million neurons lost every generation.

Daniel Graham, “Why the brain is shrinking, and what it means for us” at Psychology Today (June 17, 2021)

Graham thinks that the lower brain mass is a benefit: “Having fewer neurons, along with less body mass, means lower energy costs. All else being equal, this means less time finding food.” No doubt a leaner machine is a benefit. But perhaps the principle thing to see here is that the human mind is not simply the brain. The human mind can get by on comparatively little brain, just enough to stay connected.

Third — the question we’ve been waiting for — could we engineer a bigger brain if we wanted to? The experiment has been tried in fish. The results were not spectacular:

These studies have produced curious results. In a 2013 study of guppies, selective breeding over just a few generations produces large-brained individuals with 5-10% more brain mass than those bred to have smaller brains.

Daniel Graham, “Can a bigger brain be engineered?” at Psychology Today (July 29, 2021)

When the aquarium fish were subjected to food-finding tests, larger-brained females did better than smaller-brained females but there was no difference in the males. And later experiments showed that brain size made little difference to finding mates. In any event, male and female guppies differ much more than male and female humans so even if one wanted to stress the differences, it is unclear that the effect would be the same.

With no clear advantage identified in nature, would we still want to experiment with enabling bigger brains in humans?

Judging from research studies, Graham thinks that we can better improve our intelligence by living in an enriched and innovative environment that challenges our thinking skills.

About his recent book: “The computational neuroscientist Daniel Graham offers an innovative paradigm for understanding the brain. He argues that the brain is not like a single computer—it is a communication system, like the internet. Both are networks whose power comes from their flexibility and reliability. – Columbia University Press, Publisher”

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and

Do larger brains make us human? Is that all? Brain organoid studies suggest a “key genetic switch” that makes human brains grow larger than ape brains. Some researchers believe that our diet led to a larger brain but they differ as to which food was the ultimate brain booster. Are we missing something here?