Legal vs. quantified definitions of standards of proof.

Abstract

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.

The burden of social proof: Shared thresholds and social influence.

“…Interestingly, MacCoun (1984) found that assigned standards of proof did not have their expected effect on individual predeliberation verdict preferences. Similarly, Kagehiro and Stanton (1985) failed to find any effect of traditional verbal standard-of-proof instructions on individual mock jurors' verdicts, although they did find an effect of quantified instructions (e.g., greater than 50% for preponderance of evidence). Thus, the asymmetries shown in Table 8 illustrate that even legal standards of proof operate more like social norms than like epistemological inferences.…”

“…When mock juries are instructed to use the symmetrical "preponderance of evidence" standard, the asymmetry disappears. Interestingly, traditional "reasonable doubt" instructions have little effect on individual jurors' predeliberation judgments (Kagehiro & Stanton, 1985;MacCoun & Kerr, 1988;cf. Kerr et al, 1976).…”

Factors Affecting the Use of Naked Statistical Evidence of Liability

“…Wells also argued that these data cannot be explained according to the dominant decision model in this area, the probability threshold model. According to this model, "triers of fact" reason from evidence to fact; that is, they make an affirmative decision, such as liability, when the subjective probability of liability exceeds a threshold probability (e.g., Carlson & Dulaney, 1988;Connolly, 1987;Dane, 1985;Kagehiro & Stanton, 1985;Kerr et al, 1976;Nagel, 1979;Ostrom, Werner, & Saks, 1978;Simon, 1970;Thomas & Hogue, 1976). Wells proposed, instead, that people occasionally engage in fact-to-evidence reasoning-that is, Wells argued, "In order for evidence to have a significant impact on people's verdict preferences, one's hypothetical belief about the ultimate fact must affect one's belief about the evidence" (p. 746).…”

Preponderance of the Evidence versus Intime Conviction: A Behavioural Perspective on a Conflict Between American and Continental European Law

“…To many jurors "jury instructions are like foreign movies without subtitles" (Wald 1993:111, quoting Stephen Adler). Verbal instructions have misled subjects in mock juries (Kagehiro and Stanton 1985). Estimates of what "beyond a reasonable doubt" meant ranged from 51 to 91% of certainty (Hastie 1993:101-108).…”