When people are asked to assign probabilities to a set of outcomes, they often add up to more than 1, or 100%, violating the norms of additivity. The present research investigated the role of numeracy in additivity neglect. Study 1 found that highly numerate participants exhibited less additivity neglect than the less numerate participants. Study 2 sought to replicate the findings from Study 1 while exerting more control over the numeracy test. This however led participants to become less additive. In Study 3 the numeracy test was given before the probability tasks in Condition 1, and after the probability tasks in Condition 2. The results showed that only the more numerate participants benefited from receiving the numeracy test first. Apparently, in order to give additive responses one not only needs a certain level of mathematical skills, one also needs to be reminded of using them. The results are discussed within a framework of class-based and case-based thinking.