A recent trend in information retrieval systems technology is the development of on-line information retrieval systems. One objective of these systems has been to attempt to enhance decision effectiveness by allowing users to preferentially seek information, thereby facilitating the reduction or elimination of information overload. Since information systems users may preferentially seek information to confirm their initial beliefs, decision making effectiveness may be dependent on the accuracy of the decision maker's initial hypothesis of causality. The basic research question addressed in this paper is: Will the use of a knowledge-based DSS (KBDSS), designed to search for and present both confirming and disconfirming evidence, result in enhanced decision effectiveness? To assess the effect of information retrieval system type on decision effectiveness, a laboratory experiment was conducted in which participants were required to make an initial attribution of causality for a problem, to query either a conventional on-line information retrieval system or a KBDSS for additional information, and then to make a final attribution of causality. The conclusions reached from this experiment provide constructive guidance for information systems designers in overcoming the concept known as confirmation bias, that tendency to seek information that confirms the user's first impression.