NCJ Number
177869
Journal
Law and Human Behavior Volume: 23 Issue: 2 Dated: April 1999 Pages: 159-184
Date Published
1999
Length
26 pages
Annotation
This paper reviews recent findings about how decision makers evaluate DNA and other biological profile evidence, suggests a novel way to describe these emerging findings, and reports the results of a study designed to show the usefulness of this description.
Abstract
A small body of research has examined how decision makers (undergraduates, adult jury-eligible community members, and adults summoned for jury duty who are waiting to be selected) evaluated probability estimates associated with biological profile evidence such as blood typing, hair fibers, and DNA. The results of these studies have been described in terms of "error- based" descriptions. The error-based description posits that jurors are imperfect fact finders who predictably misuse probabilistic DNA evidence because they commit logical or mathematical errors, such as incorrectly aggregating separately presented probabilities and affording probabilistic evidence less weight than Bayesian norms would indicate. In addition to making logical or mathematical errors, this paper proposes that jurors may also be influenced by their background beliefs about the possibility of laboratory errors and intentional tampering. The study reported in this paper tested several hypotheses about the extent to which juror reactions to DNA evidence are guided by both errors and expectancies. A total of 219 jury-eligible undergraduate psychology students participated in the study. Students participated in sessions of 7-15 people and were randomly assigned to experimental conditions within each session. All participants were told that they would be reading a two-page crime scenario in which the defendant was on trial for sexual assault. An expert witness testified that DNA extracted from semen stains found on the victim matched the defendant's own DNA. Participants then read one of seven versions of the criminalist's expert testimony, which conveyed information about the source of the probability estimate (RMP or LE) or both, depending on the experimental condition. Finally, participants completed a questionnaire that included a variety of manipulation checks and dependent measures. Consistent with the error-based description, study participants incorrectly aggregated separately presented probabilities and afforded probabilistic evidence less weight than would be expected by applying Bayesian norms. Consistent with the expectancy-based descriptions, participants' background beliefs about the possibility of laboratory errors and intentional tampering affected the weight participants afforded a DNA match report. The paper discusses potential implications of these findings for the legal system and suggests directions for future research. 1 table, 1 figure, and 44 references