Who Matches? Propensity Scores and Bias in the Causal Effects of Education on Participation

In a recent study, Kam and Palmer (2008) employ propensity score matching to assess whether college attendance causes participation after reducing selection bias due to pre-adult factors. After matching the authors find no correlation, upending a major pillar in political science. However, we argue that this study has serious flaws and should not be the basis for rejecting the traditional view of an “education effect” on participation. We match on 766,642 propensity scores and use genetic matching to recover better matches with lower covariate imbalances. We consistently find positive effects as covariate balance improves, though no matching approach yields unbiased results. We demonstrate that selection is a serious concern in studying the participatory effects of college attendance and that balance in the covariates and robustness to sensitivity diagnostics should be the ultimate guide for conducting matching analyses.