No one knows exactly what benefit early humans got out of their comparatively enormous brains. Good theories suggest various possibilities—perhaps our pumped-up brains made us better foragers, hunters, cooperators, or romantic partners. However, it's clear that the brain first evolved for survival and reproduction and has been thoroughly co-opted by the modern world, where it's used for distinctly non-life-or-death activities like chess, computer games, and existential Swedish movies.

This is important, because the human brain's way of reasoning is shaped by the needs of its ancient environment, and its occasional failures in the modern world are a legacy of that design. Thousands of years ago, every decision a human made had to be quick and was based on partial facts and second-hand information. So it's no wonder that we developed the perfect tool for making snap judgments with partial facts and second-hand information—namely, common sense.