The cortex is the outer covering of the brain, the curly gray stuff you see in pictures of brains. It’s a tenth of an inch thick and covers the brain like a sheet. The prefrontal cortex, which sits behind the forehead, is just one part of the overall cortex. The last major brain region to develop during human evolutionary history, it is a measly 4 to 5 percent of the volume of the rest of the brain.
