In order to explain the apparent violation of a causal assumption, people often posit hidden causes. The assumption of independent causal influence states that the power of a cause to produce or prevent an effect is independent of other causes. Some preventers violate independent causal influence; we conducted an experiment to test whether people posit a hidden mediating cause to explain these preventers. The results indicated that participants are more likely to posit a hidden mediator when the preventer violates independent causal influence.