Persuasion is a powerful tool. Attorneys use it to win cases. Politicians practice the art to harvest votes. Salespeople and advertisers rely on it to sell everything from soap to skyscrapers. Preachers weave persuasion into their sermons. Parents try to motivate children and children try to influence their parents. Persuasion can help or hurt. It can help people land a job, earn a living, or get a friend through a tough time. But it can also make...