How individuals reason deductively has concerned researchers for many years. Yet, it is still unclear whether, and if so how, participants’ reasoning performance changes over time. In two test sessions one week apart, we examined how the syllogistic reasoning performance of 100 participants changed within and between sessions. Participants’ reasoning performance increased during the first session. A week later, they started off at the same level of reasoning performance but did not further improve. The reported performance gains were only found (...) for logically valid, but not for invalid syllogisms indicating a bias against responding that ‘no valid conclusion’ follows from the premises. Importantly, we demonstrate that participants substantially varied in the strength of the temporal performance changes and explored how individual characteristics, such as participants’ personality and cognitive ability, relate to these interindividual differences. Together, our findings contradict common assumptions that reasoning performance only reflects a stable inherent ability. (shrink)