Do people believe combined hazards can present synergistic risks?

The risk attributable to some hazard combinations can be greater than the sum of the risk attributable to each constituent hazard. Such “synergistic risks” occur in several domains, can vary in magnitude, and often have harmful, even life-threatening, outcomes. Yet, the extent to which people believe that combined hazards can present synergistic risks is unclear. We present the results of two experimental studies aimed at addressing this issue. In both studies, participants examined synergistic and additive risk scenarios, and judged whether these were possible. The results indicate that the proportion of people who believe that synergistic risks can occur declines linearly as the magnitude of the synergistic risk increases. We also find that people believe, despite scientific evidence to the contrary, that certain hazard combinations are more likely to present additive or weakly synergistic risks than synergistic risks of higher magnitudes. Furthermore, our findings did not vary as a simple function of hazard domain(health vs. social), but varied according to the characteristics of the specific hazards considered(specified vs. unspecified drug combinations). These results suggest that many people’s beliefs concerning the risk attributable to combined hazards could lead them to underestimate the threat posed by combinations that present synergistic risks, particularly for hazard combinations that present higher synergistic risk magnitudes. These findings highlight a need to develop risk communications that can effectively increase awareness of synergistic risks.