How do we make decisions and exercise judgment? We might expect that these are uniquely human activities, among the things that distinguish us from our non-human primate relatives. But Santos is pursuing a different notion — namely, that some aspects of the irrational decision-making that human adults are famous for might be shared with children and monkeys. Santos thinks this can teach us something about the origin of basic human irrationalities, such things as loss aversion, cognitive dissonance, or the anchoring bias — favoring some information over other considerations when making a decision. Why would evolution have allowed such things to appear so early in human development, when they seem to be a source of trouble and bad decisions?