When millions of Americans abandoned smoking in the 1980s, many health experts and social scientists thought they had tobacco on the run. But in the '90s progress began to slow: From 1990 to 2003, according to federal figures, only 3% of Americans gave up their cigarettes. The slowdown prompted many experts to conclude that most of the smokers who could easily quit had already done so. What remained was a hard-core group of Americans who continued to puff away despite significant health risks and severe social stigma. So social scientists turned to a new quarry: understanding the mind of the...