The factor that is not directly associated with the decline in crime since the early 1990s is "Elimination of arsenic in paint and building materials." While the elimination of arsenic in paint and building materials is a positive development for public health, it is not directly linked to the decline in crime rates. The other factors mentioned, such as the legalization of abortion, decline in the crack cocaine market, and increase in the number of police, have been studied and recognized as potential contributors to the decline in crime rates during that period.