Not having two fingers in front or having two fingers in front. That is the measure, about four centimeters depending on the size of the jointed appendages of the hand, that separates people who are of little understanding or judgment from those who have common sense. Fortunately, this is so only figuratively, an idea embodied in an adverbial locution that we use recurrently. But what is the origin of this expression?

Knowing how the brain works remains one of the greatest scientific challenges. When asked whether the larger the brain is, the greater the intelligence, some recent studies suggest that it is not. But there are also experts, such as neuroscientist Karl Zilles, who point out that people with larger brains are more likely to be more intelligent than those with smaller brains, giving wings to the old belief that babies with bigger heads they will have a brighter academic future.

Now, the brain and its dimensions are one thing and the size of the parts of the skull is another. In the 1800s, phrenology was a pioneering pseudoscience in deciding who was bad or good or what a person’s abilities and personality were with a simple external examination of the skull. Franz Joseph Gall, the father of the theory, held that a prominent forehead, at the height where the “perceptive organs” were located, indicated a superior intellect. And, conversely, a less broad forehead than usual corresponded to less intelligent people.

Only a few years later, Charles Darwin based himself on craniology (a discipline that studies the measurement of the skull) to put together part of his theory of evolution in the book The Origin of Species (1859). For the English naturalist, the frontal part of the skull was “the seat of the intellectual faculties.” Of course, with these premises Darwin classified the different “races”, placing the Europeans, with a larger skull, with greater intellectual capacity compared to Asians and Australian aborigines, with a smaller skull size. This misperception was very popular during the first half of the 20th century.

Contributions without much scientific evidence such as those of Gall or Darwin served to leave in the collective imagination the idea that less broad foreheads belonged to lazy and reckless people. Thus, at some point in the last two centuries, the expression “not having half a brain” arose to indicate someone who was not very clever.

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