Leniency Bias

It is well established that if a verdict option is favored by a substantial (e.g., two-thirds) majority of jurors prior to deliberation, the jury is very likely to ultimately reach that verdict. A number of studies have suggested the following qualification to this simple rule—in criminal juries, pro-acquittal factions tend to be more influential than proconviction factions of comparable size. The net effect of this asymmetry is a tendency for juries to be more lenient than individual jurors, except for cases that produce a large majority of jurors for conviction. This tendency constitutes the leniency bias.

Evidence for the Leniency Bias

The initial and strongest evidence for leniency bias comes from a number of jury simulation studies. Robert MacCoun and Norbert Kerr meta-analytically combined the results of 13 such studies and reported that (a) on average, acquittal was about four times as likely as conviction for mock juries that began deliberation evenly split (e.g., 6 G vs. 6 NG); (b) an initial two-thirds majority favoring acquittal was more likely to ultimately prevail (94% of the time, on average) than a two-thirds majority favoring conviction (67% of the time); and (c) the stronger the evidence against a defendant, the weaker was this bias. On the other hand, a handful of surveys of ex-jurors from actual criminal jury trials (e.g., by Dennis Devine and his colleagues) have suggested either no such asymmetry or even the reverse effect (i.e., a harshness bias), but at present it remains unclear whether or not actual criminal juries do exhibit a leniency bias. This is because there are a number of potentially important methodological ambiguities clouding the comparisons of the mock versus actual juries. For example, the surveys of actual jurors all appear to have treated jurors who say that they are undecided at the first jury vote as advocates for acquittal, which is likely to overestimate the true size of the pro-acquittal faction in the jury. In summary, there is good evidence of a leniency bias in mock juries where estimates of pro- and anticonviction faction sizes are based on direct assessment of mock jurors’ predeliberation verdict preferences in relatively close cases. There is currently no strong evidence of such a bias (and some evidence to the contrary) where these estimates are based on ex-jurors’ retrospective recollections of the number of proconviction votes at their actual jury’s first ballot, in convenience samples of diverse cases.

Explaining the Leniency Bias

One explanation for the leniency bias is the existence of a prodefendant norm among jurors. Research on group decision making and polarization suggests that one effect of group deliberation is to increase commitment to shared norms. The more consistent evidence for a leniency bias among mock jurors, who are usually college students, than among actual jurors could be interpreted as reflecting different norms in the student and nonstudent populations. A direct comparison of the leniency bias for a student versus a nonstudent sample has shown that nonstudents exhibit a somewhat weaker leniency effect, but the difference was not statistically significant.

Another, related explanation is based on the common law’s aversion to false conviction. Such values are reflected in several elements of the law, including the presumption of innocence, the prosecution’s burden of proof, and particularly the reasonable-doubt standard of proof. The law prescribes that juries must give a criminal defendant the benefit of any reasonable doubt. This should give advocates of acquittal an advantage over advocates of conviction during jury deliberation. For example, jurors favoring acquittal need only raise a single reasonable doubt in the minds of proconviction jurors, whereas jurors arguing for conviction must refute all reasonable doubts in the minds of pro-acquittal jurors. This explanation predicts that there should be no leniency bias when jurors apply a standard of proof that is not slanted to favor the defendant, such as the “preponderance of evidence” standard used in civil trials; this prediction has been confirmed experimentally. A model presented by Norbert Kerr, Robert MacCoun, and Geoffrey Kramer generalizes the asymmetry effect, demonstrating how any shared local norm can create disproportionate influence for one side of an issue.

Implications of the Leniency Bias

The leniency bias has a number of interesting implications, both for the development of psychological theory and for legal application. Asymmetries in the power of opposing factions, such as the leniency bias, have been used to analyze the group decision-making process and thereby, to better understand exceptions to the “majority-wins” rule and to predict when and why groups differ from individuals in their susceptibility to a variety of judgmental biases. The most direct applied implication of the leniency bias is that, except for cases with very strong evidence against the defendant, deliberating juries should be more likely to acquit than individual triers of fact (e.g., a judge in a bench trial). Thus, the leniency bias provides an alternative explanation for a classic finding from the landmark product of the Chicago Jury Project, The American Jury—most verdict disagreements between juries and judges were instances in which the jury was more lenient (i.e., more likely to acquit) than the judge. Harry Kalven and Hans Zeisel attributed this to differences in what judges and jurors value or know (e.g., knowledge of prior convictions). But the leniency bias suggests that this effect may stem not from who makes the decision (judges vs. jurors) but from how the decision is made (i.e., individual vs. group decision making). This interpretation suggests that if panels of judges were the triers of fact, they would likewise tend to be overall more lenient than individual judges. The fact that, in The American Jury, there was no such asymmetry in the disagreements of judges and juries for civil cases (where a symmetric standard of proof is applied) further supports this interpretation.


  1. Devine, D. J., Olafson, K. M., Jarvis, L. L., Bott, J. P., Clayton, L. D., & Wolfe, J. (2004). Explaining jury verdicts: Is leniency bias for real? Journal of Applied Social Psychology, 34(10), 2069-2098.
  2. Kerr, N. L., MacCoun, R., & Kramer, G. (1996). Bias in judgment: Comparing individuals and groups. Psychological Review, 103, 687-719.
  3. MacCoun, R. J., & Kerr, N. L. (1988). Asymmetric influence in mock jury deliberation: Jurors’ bias for leniency. Journal of Personality and Social Psychology, 54, 21-33.

Return to the overview of Trial Consulting in Forensic Psychology.