Here's a reason hit songs offer such guilty pleasure--they're designed that way. Over the last two decades a new type of hit song has emerged, one that is almost inescapably catchy. Pop songs have always had a "hook," but today's songs bristle with them: a hook every seven seconds is the rule. Painstakingly crafted to tweak the brain's delight in melody, rhythm, and repetition, these songs are highly processed products. Like snack-food engineers,...