People often prefer simpler explanations, defined as those that posit the presence of fewer causes (e.g., positing the presence of a single cause, Cause A, rather than two causes, Causes B and C, to explain observed effects). Here we test one hypothesis about the mechanisms underlying this preference: that people tend to reason as if they are using “agnostic” explanations, which remain neutral about the presence/absence of additional causes (e.g., comparing “A” vs. “B and C”, while remaining neutral about the status of B and C when considering “A”, or of A when considering “B and C”), even in cases where “atheist” explanations, which specify the absence of additional causes (e.g., “A and not B or C” vs. “B and C and not A”), are more appropriate. Three studies with US-based samples (total N = 982) tested this idea by using scenarios for which agnostic and atheist strategies produce diverging simplicity/complexity preferences, and asking participants to compare explanations provided in atheist form. Results suggest that people tend to ignore absent causes, thus overgeneralizing agnostic strategies, which can produce preferences for simpler explanations even when the complex explanation is objectively more probable. However, these unwarranted preferences were reduced by manipulations that encouraged participants to consider absent causes: making absences necessary to produce the effects (Study 2), or describing absences as causes that produce alternative effects (Study 3). These results shed light on the mechanisms driving preferences for simpler explanations, and on when these mechanisms are likely to lead people astray.