Dateline: August, 2008, Issue 2

When does physical and character evidence dilute information diagnostic of guilt?

Ideally, jurors asked to render a verdict in a criminal trial make judgments of guilt or innocence based solely on the trial evidence, which is presumably diagnostic of guilt or innocence. Information diagnostic of guilt or innocence includes eyewitness accounts, physical evidence, and the existence of an alibi.

How does information not diagnostic of guilt or innocence - a defendant's physical attributes or character - affect verdicts? Can such non-diagnostic evidence dilute information diagnostic of guilt?

Zukier and Jennings (1983-1984) examined the influence of diagnostic and non-diagnostic information on judgments of guilt in a murder trial. One group of jurors considered only information that was diagnostic of guilt (e.g., direct evidence). A second group of jurors considered this same information and also received non-diagnostic information that was "typical" about a defendant's height and vision: these jurors learned the defendant was of average height and had average vision. A third group of jurors considered the same diagnostic information, and received "atypical" information about the defendant's height and vision: the defendant was extremely tall and had extremely good vision. "Atypical" diagnostic information had no effect on jurors' verdicts. However, jurors given "typical" non-diagnostic information were more likely to acquit the defendant. The researchers concluded that "extremeness" in one category (a defendant's height and vision) is related to "extremeness" in another category (a defendant's likely guilt) and that "typicality" in one category (a defendant's height and vision) is related to "typicality" in another category (a defendant's innocence).

Smith, Stasson and Hawkes (1998-1999) found that non-diagnostic character evidence only affects verdicts when diagnostic evidence is relatively scarce. In their research, when there was a lot of diagnostic evidence pointing toward guilt, character evidence had no influence on jurors' verdicts. When there was only a small amount of diagnostic information pointing toward guilt, non-diagnostic information about a defendant that was "typical" (i.e., showing the defendant was not the sort of person who would be likely to commit the alleged crime of robbery) greatly reduced guilty verdicts. The impact of the character evidence was greatest when evidence diagnostic of guilt comprised only 20% of the total evidence presented to jurors. The researchers concluded that non-diagnostic information such as evidence of a defendant's character is more likely to influence verdicts when the prosecution's evidence is weak or ambiguous.

A defendant's characteristics and qualities, if typical, can reduce guilty verdicts when the prosecution's evidence is weak or ambiguous. When the prosecution's case is strong, these extralegal and non-diagnostic factors do not influence jurors' verdicts.

Source Zukier, H. & Jennings, D. L. (1983-1984). Nondiagnosticity and typicality effects in prediction. Social Cognition, 2, pp. 187-198.

Source Smith, H. D., Stasson, M. F., & Hawkes, W. G. (1998-1999). Dilution in legal decision making: Effect of non-diagnostic information in relation to amount of diagnostic evidence. Current Psychology, 17, pp. 333-345.