In this dissertation, I propose that, post-action, people tend to be more optimistic about outcomes when their actions were based on malleable (vague) information compared to when their actions were based on unmalleable (precise) information. However, pre-action, no such difference occurs. I term this inconsistency in optimism in the pre and post-action stage, the Information Malleability Effect (IME). These actions could include the choice of a product, drawing a ball from an urn, or consumption of a food item. Prior research on ambiguity aversion has reliably documented that people are generally averse to making decisions based on malleable information. On the other hand, research on situated optimism has demonstrated that people exhibit a high level of optimism for events they consider more desirable and they distort the available information to make the desirable events seem more likely to occur. I review these two streams of literature and show that although both literatures make predictions in either the pre or the post-action stage, neither of them alone can explain the IME. I propose a theoretical framework to explain the underlying cause of the IME that combines these two streams of literature and utilizes the motivated reasoning account. Based on this framework, I posit hypotheses that are tested across a series of experiments. Experiment 1a and 1b demonstrate the IME in a between and within participant design. Experiment 2 demonstrates that interpretational flexibility of malleable information results in positive outcomes appearing more plausible and negative outcomes less plausible compared to when information is unmalleable. Experiment 3 provides support for the proposed underlying process by priming accuracy and desired goals.