The scalar approach to negative polarity item (NPI) licensing assumes that NPIs are allowable in contexts in which the introduction of the NPI leads to proposition strengthening (e.g. Kadmon & Landman 1993; Krifka 1995; Lahiri 1997; Chierchia 2006). A straightforward processing prediction from such a theory is that NPIs facilitate inference verification from sets to subsets. Three experiments are reported that test this proposal. In each experiment, participants evaluated whether inferences from sets to subsets were valid. Crucially, we manipulated whether the premises contained an NPI. In Experiment 1, participants completed a metalinguistic reasoning task and Experiments 2 and 3 tested reading times using a self-paced reading task. Contrary to expectations, no facilitation was observed when the NPI was present in the premise compared to when it was absent. In fact, the NPI significantly slowed down reading times in the inference region. Our results therefore favour those scalar theories that predict that the NPI is costly to process (Chierchia 2006), or other, non-scalar theories (Ladusaw 1992; Giannakidou 1998; Szabolcsi 2004; Postal 2005) that likewise predict NPI processing cost but, unlike Chierchia (2006), expect the magnitude of the processing cost to vary with the actual pragmatics of the NPI.