In this fascinating history of the lie detector, Ken Alder exposes some persistent truths about our culture: why we long to know the secret thoughts of our fellow citizens; why we believe in popular science; and why we embrace "truthiness." For centuries people searched in vain for a way to unmask liars, seeking clues in the body's outward signs: in blushing cheeks and shifty eyes. Not until the 1920s did a cop with a PhD team up with an entrepreneurial high school student and claim to have invented a foolproof machine capable of peering directly into the human heart. Scientists repudiated the technique, and judges banned its results from criminal trials, but in a few years their polygraph had transformed police work, seized headlines, and enthralled the nation. In this book, Alder explains why America-and only America-has embraced this mechanical method of reading the human soul. Over the course of the twentieth century, the lie detector became integral to our justice system, employment markets, and national security apparatus, transforming each into a game of bluff and bluster. The lie detector device may not reliably read the human mind, but this lively account shows that the instrument's history offers a unique window into the American soul. Ken Alder is a professor of history and the Milton H. Wilson Professor in the Humanities at Northwestern University. He is the author of The Measure of All Things: The Seven-Year Odyssey and Hidden Error that Transformed the World and Engineering the Revolution.