Although recent research has found that neurological expert testimony is more persuasive than other kinds of expert and nonexpert evidence, no impact has been found for neuroimages beyond that of neurological evidence sans images. Those findings hold true in the context of a mens rea defense and various forms of insanity defenses. The present studies test whether neuroimages afford heightened impact in the penalty phase of capital murder trials. Two mock jury experiments (n = 825 and n = 882) were conducted online using nationally representative samples of persons who were jury eligible and death qualified. Participants were randomly assigned to experimental conditions varying the defendant's diagnosis (psychopathy, schizophrenia, normal), type of expert evidence supporting the diagnosis (clinical, genetic, neurological sans images, neurological with images), evidence of future dangerousness (high, low), and whether the proponent of the expert evidence was the prosecution (arguing aggravation) or the defense (arguing mitigation). For defendants diagnosed as psychopathic, neuroimages reduced judgments of responsibility and sentences of death. For defendants diagnosed as schizophrenic, neuroimages increased judgments of responsibility; nonimage neurological evidence decreased death sentences and judgments of responsibility and dangerousness. All else equal, psychopaths were more likely to be sentenced to death than schizophrenics. When experts opined that the defendant was dangerous, sentences of death increased. A backfire effect was found such that the offering party produced the opposite result than that being argued for when the expert evidence was clinical, genetic, or nonimage neurological, but when the expert evidence included neuroimages, jurors moved in the direction argued by counsel.

Recent developments in the neuropsychology of criminal behavior have given rise to concerns that neuroimaging evidence (such as MRI and functional MRI [fMRI] images) could unduly influence jurors. Across four experiments, a nationally representative sample of 1,476 jury-eligible participants evaluated written summaries of criminal cases in which expert testimony was presented in support of a mental disorder as exculpatory. The evidence varied in the extent to which it presented neuroscientific explanations and neuroimages in support of the expert's conclusion. Despite suggestive findings from previous research, we found no evidence that neuroimagery affected jurors' judgments (verdicts, sentence recommendations, judgments of the defendant's culpability) over and above verbal neuroscience-based testimony. A meta-analysis of our four experiments confirmed these findings. In addition, we found that neuroscientific evidence was more effective than clinical psychological evidence in persuading jurors that the defendant's disorder reduced his capacity to control his actions, although this effect did not translate into differences in verdicts.

The introduction of neuroscientific evidence in criminal trials has given rise to fears that neuroimagery presented by an expert witness might inordinately influence jurors' evaluations of the defendant. In this experiment, a diverse sample of 1,170 community members from throughout the U.S. evaluated a written mock trial in which psychological, neuropsychological, neuroscientific, and neuroimage-based expert evidence was presented in support of a not guilty by reason of insanity (NGRI) defense. No evidence of an independent influence of neuroimagery was found. Overall, neuroscience-based evidence was found to be more persuasive than psychological and anecdotal family history evidence. These effects were consistent across different insanity standards. Despite the non-influence of neuroimagery, however, jurors who were not provided with a neuroimage indicated that they believed neuroimagery would have been the most helpful kind of evidence in their evaluations of the defendant.

No abstract

The impact of forensic science evidence on jurors' judgments is critically important to the criminal justice system. The assignment of low or high weight to such testimony can be the difference between acquittal or conviction. Many of the traditional forensic sciences (e.g., fingerprints and bitemarks) draw their strength largely from the subjective judgments of examiners who testify about whether evidentiary prints or other markings are consistent with (or "match") known markings from a person or object. In an online experiment (Experiment 1) and a realistic jury simulation using actual jurors or jury-eligible adults (Experiment 2), this article investigates 3 factors that might affect how jurors think about and use forensic science evidence. These factors are (a) whether the forensic science method had been scientifically tested, (b) the forensic scientist's background and experience, and (c) the sophistication of the forensic science technology. The results show a strong and consistent effect for examiner background and experience on evidence strength judgments, no effect for forensic technology sophistication, and a limited and inconsistent effect for scientific testing (present in the online experiments, absent in the realistic jury simulation). These findings raise concerns about potential undue influence of examiner background and experience on jurors' judgments and lack of clear influence of scientific testing. The implications of our findings for criminal justice practices and policies are considered.