This paper investigates the effects of technical knowledge and decision aid use on financial statement fraud risk assessments made by directors and students. More extreme fraud risk assessments are made when participants identify and process larger (smaller) numbers of diagnostic (non-diagnostic) factors, with technical knowledge driving diagnostic factor identification. Significant decision aid-technical knowledge effects are also found; decision aid use has a detrimental effect on high-knowledge directors while improving performance in inexperienced, low-knowledge students. These results suggest that although decision aids can afford gains in performance in inexperienced users, they can have unintended and/or paradoxical behavioural effects on experienced users. Copyright (c) 2008 The Authors. Journal compilation (c) 2009 AFAANZ.