This research tested and extended a laboratory-derived model of the origins of attitude certainty using a real attitude object: Teaching children with attention-deficit/hyperactivity disorder (ADHD). In Study 1, an experiment manipulated the amount of information, thought, and consistency of information delivered to pre-service teachers (n = 224) with no prior experience with ADHD. Structural equation modelling (SEM) of Study 1 did not support the original model, in which relationships between attitude certainty and objective knowledge, thought, and consistency were mediated by perceived knowledge, thought, and ambivalence. Instead, objective amount of information, thought, and consistency interacted in their effect on attitude certainty. Study 2 (n = 368) used a survey to test whether experiences with ADHD (personal, direct and indirect) among in-service and pre-service teachers were antecedents of attitude certainty, and to test perceived accessibility as a mediator. SEM supported both these hypotheses. Perceived accessibility and perceived knowledge mediated the relationship between attitude certainty and prior experiences with ADHD, and between attitude certainty and objective knowledge. Together, the results suggest that psychological processes underlying strong attitude certainty differ according to the familiarity and personal relevance of the attitude object, and the context and stage of attitude formation. The results have practical utility for teacher training at pre-service and in-service levels.