Mounting evidence that certain hazard combinations present synergistic risks for adverse outcomes, including violent crime, cancer, and species extinction, highlights the importance of understanding the risk attributable to combined hazards. However, previous studies indicate that individuals often misjudge synergistic risks as additive or sub-additive risks, and there is little research that explores the cognitive reasoning that may lead individuals to make such judgements. This study aims to fill this gap. Participants were asked to review several scenarios that described the risk magnitude presented by a combined hazard. They were required to judge whether each scenario was possible and to explain the reasoning that led to their judgement. The results show that many participants demonstrated an awareness of synergistic risk and that their reasoning was typically characterized by rudimentary knowledge of an underlying causal mechanism for the increased risk (e.g., a chemical reaction between drugs). Conversely, several participants adopted a line of reasoning that precluded the concept of synergistic risk. Many of these participants appeared to employ an additive model of risk, corresponding to the notion of ‘adding’ one hazard to another. Contrary to much previous research, we found little evidence to indicate that people tend to employ a sub-additive model of risk for combined hazards. Implications for future research and the improvement of risk communications concerning synergistic risks are discussed
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