We make decisions every day. Most of them are small: Should I buy that shirt? Others demand more thought: Is marriage right for me? The common thread running between all of them is that we are unequipped to make sense of any of it. The world in which the human race came of age—one of ferocious predators and unforgiving nature—is no longer the world we live in. For the risks we face now, we are out of date. Dan Ariely, author of bestsellers like Predictably Irrational, has built a career mapping the peculiarities of our innermost decision-making foibles, and offers insight in guarding against them.