Scientific reasoning skills have been defined as mental processes used when engaged in scientific inquiry. Research aimed to develop and validate a Scientific Reasoning in Medicine (SRM) instrument through a psychometric approach which included a preliminary phase with 60 medical students and physicians, followed by a revision phase and subsequent research with 209 medical students and physicians. Research focused on determining the extent to which item content contributed significantly to explaining the variance in SRM, if the level of scientific reasoning differed in relation to medical expertise and if individuals who were inclined to a more rational thinking style showed higher scientific reasoning. Results indicated that item content explained 47% of the variance in SRM, there were significant differences in scientific reasoning depending on expertise and participants who scored higher on the Cognitive Reflection Test and the Need for Cognition scale, also scored higher on the SRM instrument.