Three laboratory experiments were conducted to compare legal (unquantified) definitions of three standards of proof ("preponderance of the evidence," "clear and convincing evidence," and "'beyond a reasonable doubt") with quantified definitions, in which the standards of proof were expressed in probability terms (51%, 71%, and 91% probability of truth). In the first experiment, the quantified definitions had their intended effect--verdicts favoring the plaintiffs decreased as the standard of proof became stricter--but the legal definitions did not have their intended effect. These results were replicated in the second experiment; in addition, jury instructions that combined the two types of definitions (legal and quantified) did not have their intended effect. Results of the third experiment suggest that some legal definitions may be able to communicate their intended difficulty level when they appear in a comparative context.