INTRODUCTION
Law has traditionally been described as grounded in reason and devoid of emotion (Bandes 1999, Maroney 2011). Yet, people naturally feel moral emotions when they witness an act of harm to another person, which is an almost definitional aspect of a legal factfinder's job when judging a criminal or civil case. Broadly, moral emotions are emotional responses to moral violations, including "other-condemning" emotions like anger and disgust, or emotions that motivate moral behavior, including "other-suffering" emotions like sympathy and empathy—among others (Haidt, 2002). Thus, negative moral emotions commonly include those with a negative valence that focus on condemning immoral acts of others (i.e., anger, disgust, moral outrage) and positive moral emotions commonly include those with a positive valence focused on moral concern for others (i.e., sympathy, empathy, pity) (Giner-Sorolla, 2012). Judges and jurors are asked to delve into the most disturbing details of whether and why someone caused intentional or unintentional (and often violent) harm to another person. Not only are emotions inevitably and inextricably linked to cognition, but to some degree they are desirable. People with brain damage to areas necessary for social emotions exhibit abnormal and sometimes disturbing moral judgment of what is right and wrong (Koenigs et al. 2007). However, extremely emotional evidence at trial can heighten factfinders' moral emotions and stress to a problematic degree. Heightened moral emotions can directly prejudice factfinders against defendants by creating a need to blame and punish for the harm they have witnessed. More insidiously, heightened moral emotions can also unconsciously bias even the most well-meaning and diligent factfinder's decision-making process in ways that can prejudice them against defendants. In addition, victims', defendants', witnesses', and attorneys' emotional expressions can bias factfinders' judgments of their credibility when they violate factfinders' (often incorrect) expectations for what emotion they think is appropriate. Further, when factfinders themselves express their emotions, they can gain or lose influence, depending on their gender or race. Drawing from moral, legal, cognitive, and social psychological science, I review how factfinders' experienced and expressed emotion can lead to biases in legal decision making, as well as both misguided and promising interventions aimed at helping factfinders regulate their emotions.
EMOTIONAL REACTIONS TO EVIDENCE AT TRIAL
Emotionally disturbing evidence, such as gruesome crime-scene photographs, is extremely common in homicide trials (Schweitzer & Nuñez 2018)—but is also likely to appear in any civil or criminal trial that involves violence or accidental injuries. With advancing technology, this is likely to include high-resolution photographs of gruesome injuries, video footage of horrific accidents and/or violent crimes, and even potentially Hollywood-level, animated reenactments of violence or accidents that give the air of evidence but do not in fact hold probative value. Jurors report exposure to graphic evidence and emotional testimony as common sources of stress (Lonergan et al. 2016), leading to vicarious trauma (Robertson et al. 2009). After serving jury duty, substantial percentages of jurors meet diagnostic criteria for post-traumatic stress disorder (Palmer 2005) and trauma-related symptoms including intrusive memories, insomnia, and nightmares (Lonergan et al. 2016). Jurors meet criteria for clinical depression more often when exposed to graphic evidence (Shuman et al. 1994). Judges also report vicarious trauma from being exposed to the details of horrific violent crimes (Chamberlain & Miller 2008, Jaffe et al. 2003) and experience anger (Maroney 2012), sadness (Anleu & Mack 2005), and fear (Maroney 2011) on the job. Not only are trials a recipe for intense moral emotions, but they officially assign factfinders as the arbiters of ensuring justice for the emotionally disturbing harms they witness.
Traditionally, being a good judge has meant remaining cold, unemotional, and impartial (Maroney & Gross 2014). Indeed, both trial attorneys and federal judges believe that judges are less likely to be influenced by their emotional reactions than are jurors (Figure 1), rating judges below the midpoint but jurors above the midpoint. Psychological research demonstrating desensitization, or the decreased sensitivity and reactivity of typical emotional responses to events (Cline, Croft & Courrier, 1973), in other contexts (Anderson et al. 2010) suggests that judges could develop blunted emotional reactions to disturbing evidence over the course of their careers. Unfortunately, very little empirical research compares judges' and jurors' emotional responses or how much they are affected by them. The psychological science I review on the impact of experiencing and expressing emotions on legal judgments is therefore largely based on lay samples. The lack of judicial research precludes me from being able to say whether it applies equally to judges and highlights the importance of future research on judges.
EXPERIENCED EMOTION AND LEGAL FACTFINDING
The Effect of Emotion on Legal Judgments
Jurors' emotional reactions are significantly related to their legal judgments (Feigenson 2016). Negative moral emotions like anger, disgust, and moral outrage, in particular, are correlated with more guilty and liability verdicts (Bright & Goodman-Delahunty 2006, Matsuo & Itoh 2017, Salerno 2017, Salerno & Peter-Hagene 2013, Salerno & Phalen 2019a) and harsher punishments (e.g., Georges et al. 2013, Nuñez et al. 2015). Positive moral emotions, such as sympathy or empathy for victims, are correlated with decreased convictions (Jones et al. 2020), whereas empathy for defendants can increase convictions (Archer et al. 1979, Haegerich & Bottoms 2000.
Emotionally evocative evidence can elicit these moral emotions in jurors, which are, in turn, associated with harsher verdicts and punishment. These experiments most commonly test the impact of gruesome photographs of victims and victim impact statements (VIS) from victims or their families (Bandes & Salerno 2014, Phalen et al. 2020, Salerno & Bottoms 2009). When mock jurors are randomly assigned to view gruesome photographs of victims' injuries, they render verdicts more in line with blaming the defendant and harsher punishments, compared with those who learn about the details of the injuries through verbal testimony (Grady et al. 2018). More specifically, gruesome photographs increase guilty verdicts (Bright & Goodman-Delahunty 2006, Edwards & Mottarella 2014, Matsuo & Itoh 2017, Salerno 2017), liability verdicts (Bright & Goodman-Delahunty 2011, Oliver & Griffitt 1976), harsher sentences (Finkelstein & Bastounis 2010), and damage awards (Oliver & Griffitt 1976, Whalen & Blanchard 1982).
Several studies have identified jurors' emotional reactions to the photographs as an explanation (or mediator) of the effect. Mock jurors who saw gruesome photographs were significantly more likely to vote guilty via increased disgust (Salerno 2017) and anger (Bright & Goodman-Delahunty 2006). Similarly, a gruesome photograph of a plaintiff's severed leg significantly increased liability verdicts via increased disgust (Salerno & Phalen 2019a). Seeing a combination of photographs of the victim alive and well before she died and gruesome postmortem photographs after she died is particularly potent. Mock jurors are even more conviction prone when they see this combination compared with just postmortem photographs, because seeing the contrast increases anger and sadness in addition to the disgust elicited by the postmortem photographs (M. Adamoli, H.J. Phalen, & J.M. Salerno, manuscript in preparation).
Victim impact statement (VIS) studies provide converging evidence that emotional responses to evidence can lead to harsher judgments. Mock jurors exposed to VIS recommended harsher sentences than those who were not exposed (Bandes & Salerno 2014, Phalen et al. 2020). At least one study identified emotional reactions as the explanation: Mock jurors who saw VIS were more likely to recommend the death penalty than those who did not see VIS, because such statements increased sympathy/empathy toward the victim (Paternoster & Deise 2011).
Beyond gruesome photographs and VIS, experiments have identified emotional reactions as an explanation for the impact of many other factors that make mock jurors choose harsher verdicts and punishments. Examples include mock jurors (a) punishing gay offenders more, because they elicit more moral outrage relative to heterosexual offenders (Salerno et al. 2014); (b) voting liable more when civil sexual assault defendants were male, because they elicited more moral outrage than female defendants (McCracken & Stevenson 2017); (c) voting guilty less when a gay panic defense was offered, because they felt less moral outrage than when they did not hear a gay panic defense (in the case of conservative jurors) (Salerno et al. 2015); and (d) voting guilty more when they saw irrelevant negative emotional pretrial publicity about the defendant, because it made them angrier than jurors who did not see it (Ruva et al. 2011).
Probative Versus Prejudicial?
The fact that emotionally disturbing evidence like crime-scene photographs or VIS affects factfinders' judgments is probably not surprising to attorneys or judges. Nor would it be concerning unless it was clear that it was affecting jurors' judgments through improper or extralegal channels. If emotional evidence affects decisions because of the information it provides, fine. If emotional evidence affects decisions because of factfinders' emotional reactions, we must scrutinize when that emotion might be probative, and when that emotion prejudices them against defendants (Bandes & Salerno, 2014), which is more problematic. The legal system recognizes the potentially prejudicial impact of emotionally disturbing evidence. Rule 403 dictates that judges can exclude relevant evidence if its prejudicial impact outweighs its probative value, clarifying in the advisory notes that prejudice "means an undue tendency to suggest a decision on an improper basis, commonly, though not necessarily, an emotional one". This would be an incredibly difficult assessment even for emotion researchers, let alone for a judge with no psychological training. She must somehow quantify the probative value of, for example, a given photograph (taking into account redundant evidence that establishes the same information), play armchair psychologist and anticipate how the photograph might prejudice decision making, and weigh the two against each other.
Emotion is not monolithically prejudicial or probative. Instead, determining whether an emotional response is probative or prejudicial is incredibly complex and case-specific. Although there is certainly a lot of empirical evidence (that will be reviewed in detail) that emotion can prejudice legal factfinders' decision making processes, emotion also has potential to be probative in some circumstances. In United States v. Old Chief, the Supreme Court recognized the prosecution's right to "tell a colorful story with descriptive richness," and some argue that stripping evidence of its emotional impact would mean stripping it of its persuasive, and therefore, probative power. For example, people who endorse a retribution-based philosophy of punishment argue that emotional reactions to harm are probative to punishment decisions—and in fact, in the death penalty context, it is made explicit that jurors should consult their emotions. Some consider moral outrage to be an emotional barometer that reflects how much harm has been committed, which determines how much punishment is appropriate (Bastian et al. 2013, Carlsmith et al. 2002, Lotz et al. 2011, Okimoto & Brescoll 2010, Salerno & Peter-Hagene 2013). Thus, factfinders' emotional reactions to harm depicted in gruesome photographs or VIS might provide probative information regarding how much punishment a guilty defendant deserves. Factfinders' moral outrage or sympathy/empathy about plaintiffs' pain or grief might be probative for quantifying pain-and-suffering damages. In some cases, jurors' emotional responses might also be probative for verdicts. For example, in self-defense cases, jurors' empathetic fear might inform whether they believe the defendant felt imminent danger, or jurors' anger when hearing evidence of provocation might inform whether they think the defendant's loss of control was reasonable.
Yet, there are also many circumstances in which jurors' emotional reactions to emotional evidence might be prejudicial in that their emotional responses can bias their decision making processes, while providing little probative value beyond what is already conveyed via other evidence. Take an example of jurors who are shown gruesome photographs of injuries while making a verdict determination about whether the evidence that a murder defendant committed the act in question is strong enough to meet the burden of proof. Their emotional reaction to seeing graphic depictions of harm or suffering (that might be probative for punishment judgments) would not be probative for a guilt determination. Yet, their emotional reaction could instigate a biased review of the evidence in a way to support voting guilty, to satisfy their emotional need to see someone punished for the harm they witnessed. If the photographs' probative information about the injuries can be established in verbal testimony or in a less emotionally evocative medium, this prejudicial impact might outweigh its probative value.