Can science step in and assist in Rule 403 determinations of “unfair prejudice”? When confronted with “gruesome” evidence, all too often autopsy photos or images of severe injuries, judges must assess whether there is a risk of unfair prejudice or misleading the jury and then, if the risk is present, “may” exclude the proof.1 Yet there is no court-dictated workable metric for assessing when either risk is present beyond boilerplate terminology such as whether the “[e]vidence… makes a conviction more likely because it provokes an emotional response in the jury or otherwise tends to affect adversely the jury’s attitude toward the defendant wholly apart from its judgment as to his guilt or innocence of the crime charged.”2 The standard is akin to that used to describe when material is obscene — an “I know it when I see it” approach to decision-making.3 This approach begets arbitrariness. That this is so may be seen by contrasting claims of unfair prejudice in criminal and civil cases. The default in criminal seems to be that of admissibility, demonstrated in the extreme …