Estimating the potential impact of nonvoters on outcomes of parliamentary elections in proportional systems with an application to German national elections from 1949 to 2009

“If turnout was 100%, would it affect the election result?” is a frequently asked research question. So far, the question has been primarily answered regarding the changes in the distribution of votes. This article extends the analysis to changes in the distribution of seats and government formation. It therefore proposes a method that factors in apportionment methods, election threshold, sizes of parliaments, leverage of nonvoters, closeness of election results, and individual characteristics of nonvoters. The method is then applied to German national elections from 1949 to 2009. The application shows that Germany's Social Democratic Party (SPD) would have gained from the counterfactual participation of nonvoters, although usually not enough to result in a government change. However, the elections of 1994 and 2005 show evidence that such a change could have happened.