Order effects in diagnostic reasoning with four candidate hypotheses

Sequentially observed symptoms in diagnostic reasoning have to be integrated to arrive at a final diagnosis. In our experiments employing quasi-medical problems, four sequentially presented symptoms were consistent with multiple diagnostic hypotheses. We tested whether symptom order creates biases in symptom evaluation. Early symptoms induced a bias towards the initial hypothesis even though an alternative hypothesis was equally supported. In two experiments, stepwise ratings were prompted to explicitly highlight alternative hypotheses. Explicit highlighting eliminated the bias towards the initial hypothesis if only two hypotheses competed, but the bias remained if more than two hypotheses were associated with symptoms in a sequence. Our results are consistent with process models of information integration that specify how early information can frame the processing of later information. Extending previous results obtained with fewer contending hypotheses, we show limits in impartially considering more than two hypotheses.