A wave of at least 180 proposed laws tightening voting rules washed over 41 statehouses in 2011 and 2012, by the count of New York University’s Brennan Center for Justice. Only a fraction of those bills passed and survived the scrutiny of the courts, but the new rules cover voters in 13 states, several quite populous, in time for next month’s election. More laws are to start afterward. Partisans and experts are arguing, over the airwaves and in the courts, about the effects of all this on voter turnout, for which few studies exist. (The most rigid voter ID laws are believed to affect about 10 percent of eligible voters, said Lawrence Norden of the Brennan Center.)