Who shows the Unlikelihood Effect - and why?

Recent work shows that people judge an outcome as less likely when they learn the probabilities of all single pathways that lead to that outcome, a phenomenon termed the Unlikelihood Effect. The initial explanation for this effect is that the low pathway probabilities trigger thoughts that deem the outcome unlikely. We tested the alternative explanation that the effect results from people's erroneous interpretation and processing of the probability information provided in the paradigm. By reanalyzing the original experiments, we discovered that the Unlikelihood Effect had been substantially driven by a small subset of people who give extremely low likelihood judgments. We conducted six preregistered experiments, showing that these people are unaware of the total outcome probability and do formally incorrect calculations with the given probabilities. Controlling for these factors statistically and experimentally reduced the proportion of people giving extremely low likelihood judgments, reducing and sometimes eliminating the Unlikelihood Effect. Our results confirm that the Unlikelihood Effect is overall a robust empirical phenomenon, but suggest that the effect results at least to some degree from a few people's difficulties with encoding, understanding, and integrating probabilities. Our findings align with current research on other psychological effects, showing that empirical effects can be caused by participants engaging in qualitatively different mental processes.