Does DNA evidence in the form of a likelihood ratio affect perceivers’ sensitivity to the strength of a suspect’s alibi?

Does DNA evidence in the form of a likelihood ratio affect perceivers’ sensitivity to the strength of a suspect’s alibi?

Forensic expert testimony is beginning to reflect the uncertain nature of forensic science. Many academics and forensic practitioners suggest that forensic disciplines ought to adopt a likelihood ratio approach, but this approach fails to communicate the possibility of false positive errors, such as contamination or mislabeling of samples. In two preregistered experiments (N1 = 591, N2 = 584), we investigated whether participants would be convinced by a strong DNA likelihood ratio (5,500 in Experiment 1 and 5,500,000 in Experiment 2) in the presence of varying alibi strengths. Those who received a likelihood ratio often concluded that the suspect was the source of the DNA evidence and guilty of the crime compared with those who did not receive a likelihood ratio—but they also tended to conclude that an error may have occurred during DNA analysis. Furthermore, as the strength of the suspect’s alibi increased, people were less likely to regard the suspect as the source of the evidence or guilty of the crime, and were more likely to conclude that an error may have occurred during DNA analysis. However, people who received a likelihood ratio were actually more sensitive to the strength of the suspect’s alibi than those who did not, driven largely by the low ratings in the strongest alibi. Interestingly, the same pattern of results held across both experiments despite the likelihood ratio increasing by two orders of magnitude, revealing that people are not sensitive to the value of the likelihood ratio.

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