As a mother, Lisa Guernsey wondered about the influence of television on her two young daughters. As a reporter, she resolved to find out. What she first encountered was tired advice, sensationalized research claims, and a rather draconian mandate from the American Association of Pediatricians: no TV at all before the age of two. But, like many parents, she wanted straight answers and realistic advice, so she kept digging: she visited infant-perception labs and child development centers around the country. She interviewed scores of parents, psychologists, cognitive scientists, and media...