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What’s your cerebrotype?

by Rowan Hooper

In “The Prince,” Machiavelli set out his manifesto of duplicity and deception. His name stands for cunning, for forming alliances with those in power. The theory of Machiavellian Intelligence proposes that with the advent of social interaction, the advantage gained by manipulating others was the driving force behind the evolution of the primate brain. The theory receives support today from a paper in Nature.

Mammals, we all know, have big brains. Brain size increases with body size, so as you move from mice to monkeys, brain size gets proportionally bigger, like moving through a Russian doll set from the inside out. But Damon Clark and colleagues at Princeton University’s departments of Molecular Biology and Physics have now found that the Russian doll analogy doesn’t describe the whole picture.

If you look at brain size within a group of mammals, such as insectivores, the analogy holds: Brain size changes in proportion to body size, and the various parts of the brain, like the cortex and the cerebellum, stay the same size in proportion to each other. However, the proportions of different parts of the brain to the total brain size change in different groups of mammals. In other words, each layer of the “doll” is not just a bigger version of the last but is sometimes architecturally different.

For example, in fruit bats, which use their eyes to guide them when they fly, the size of the cerebellum, the part of the brain involved in processing sensory data, is normal for a mammal of their body size. But among bats that use echolocation, all species have an unusually large cerebellum. This area needs to be big in bats that use radar to process the extra sensory data.

Clark and his colleagues devised a system for measuring this architectural change in brain structure. By comparing the proportional size of different parts of the brain to total brain size, they came up with what they called a “cerebrotype.” This cerebrotype was then incorporated into a wide-ranging analysis of mammalian species.

Their results show that within mammalian groups the ce- rebrotype remains fairly constant, despite large variations in brain size. So insectivores — which all do essentially the same thing, i.e., grub around for insects — all have the same cerebrotype, but the cerebrotype changes when there is a change in behavior and, therefore, a change in brain function. So echo-locating bats, which catch insects at night using radar, have a different cerebrotype to fruit bats.

But it is in the primates where there are the most dramatic changes in cerebrotype, and the scientists propose that the changes are evolutionary adaptations to an increasingly complex social life.

The outer layer of the brain’s cortex is the neocortex, a thin, wrinkled structure, the source of all our higher cognitive functions. Unfold the neocortex of a rat and it would cover the area of a stamp. That of a monkey would cover a postcard; a chimp’s a sheet of A4 paper. A human’s neocortex would cover four sheets of paper.

The enlargement of the primate neocortex relative to other brain areas was explosively fast by geological standards. It is thought to have been powered by two forces: foraging strategies and social interactions. When primates began to use tools — sticks and stones — the primate brain started to enlarge.

The social-intelligence hypothesis of primate brain evolution says that the brain enlarged to use other individuals as tools. Here’s Machiavelli.

If you can skillfully manipulate your social environment, as well as your foraging environment, you’re going to do well. As primates began to live in social groups, those individuals who could predict the outcome of interactions, and skew them toward preconceived goals . . . those early Machiavellians ruled the roost.

There are three Machiavellian routes by which intelligence is said to have developed. First, through the transmission of novel behaviors. This can be seen in Japanese macaques, who teach others in their group how to wash food before eating it. The second and third routes were through primates manipulating their social environment, aiming, uniquely, toward preconceived goals. Deception and the formation and maintenance of alliances require huge increases in brain processing power.

The complexities of group living and social interactions reached a pinnacle with modern human society. But it doesn’t mean that we are all ruled by Machiavellian cunning, in thrall to greed and lies. Some humans, at least, have moved far beyond this.

Einstein, for example, feted as the greatest genius in human history, even looks like he had a different cerebrotype to the rest of us. The inferior parietal region of Einstein’s brain — that’s the part thought to be involved in mathematical reasoning — was 15 percent wider than normal. Einstein’s reasoning went far beyond what most of us can conceive — and far beyond Machiavelli, who died after seeing his manifesto banned by the Catholic church, and after seeing Florence once again become a republic.