This book considers the impact of psychology on world events, looking at how mental illness and personality disorders have affected history. How have mental illness and personality disorders influenced history? This lively investigation demonstrates that, when conditions are ripe, one unstable individual can create the best or worst moments of a generation or even a century. Beginning with Alexander the Great, whose megalomania caused widespread bloodshed yet powerfully shaped world history through the spread of Greek culture, the author examines the various forms of mental illness among people of great influence. These includes emperors, like the Romans Caligula and Elagabalus, kings like George III of England and Charles II of Spain, and lesser known rulers such as sixteenth-century Hungarian noblewoman Elizabeth Bathory, who is in the Guinness World Records as the most prolific female serial killer of all time. In more recent times, the author considers the mental instability exhibited by dictators Adolf Hitler, Joseph Stalin, and Idi Amin, as well as female prison guard Irma Grese, whose cruelties at Auschwitz were infamous. He also discusses rumors of cognitive decline among American presidents Woodrow Wilson, Ronald Reagan, and Donald Trump, and the ways in which American democracy copes with the disability of its leaders. And he considers cases where whole societies seem to be gripped by the madness of mob rule. Ferguson concludes with an eye toward the future, considering the power of social media to amplify fringe ideas, giving extremist and outright crazy perspectives greater exposure and influence than ever before.