Here, for instance, is a chilling fact about the nineties: In any given week of the decade, there was a 10 percent chance the No.1 song was by Boyz II Men. Add Whitney Houston, Mariah Carey, and Bryan ­Adams, and chances hit 24 percent. Americans spent a quarter of a decade listening to this sort of thing: big, lavish ballads, built to charm middle-aged and middle-school listeners alike. Try to picture an environment or purpose for these songs, and the mind drifts to graduations, school-gym talent shows, Olympics montages.