The first-to-test bias: Impact of testing order on assigning responsibility for contagion.

When a contagious disease spreads, people wonder about who to blame for transmission. Herein, we document a novel bias, the "First-To-Test" bias, that emerges when individuals assign responsibility for contagion within a dyad. People tend to believe that the member of the dyad who tested positive first is more likely to have given the disease to the other member, even when all other relevant factors are held constant. That is, while using testing order as a basis for assigning responsibility for a dyad's contraction of a contagious disease may be rational in cases where all other relevant factors are not held constant, we show that individuals are more likely to allocate responsibility to whoever tested positive first even when these relevant factors are held constant. This overgeneralization bias emerges regardless of whether the evaluator is an outside observer or the member of the dyad who tested positive first. While we explore this bias with COVID-19 and strep throat, it has implications for other contagious diseases such as sexually transmitted infections (STIs) and illnesses often spread among school children (e.g., influenza, whooping cough). We conclude by discussing its implications for patients and organizations.