For over a century, people have gladly filed themselves into two camps: left-brained and right-brained. If your left brain is dominant, you’re logical and methodical. If your right brain has control of the wheel, you’re creative, emotional and artsy. 

Certain parts of the brain do control certain things. Language, for instance, is controlled by the left side. It was this discovery, made by neuroscientists Paul Broca and Carl Wernicke in the 19th century, that led to a wider Victorian obsession with “dual-brain theory.” Robert Louis Stevenson played upon it in his novella Strange Case of Dr. Jekyll and Mr. Hyde, in which the protagonists—one good, one evil—turn out to be the two personalities of one man. 

It’s perhaps unsurprising that the Victorians took to the theory s...