Police officers decide to detain and search civilians under uncertainty and risk, and both false positive and false negative errors can be costly. The courts apply the reasonable suspicion standard of proof to evaluate the constitutionality of nonconsensual stops and searches, placing an ambiguous and subjective assessment of a poorly understood psychological state at the center of laws, policies, and trainings on police-civilian contact. The law and psychology of suspicion may have important effects on the frequency, accuracy, and reporting of policing decisions. Investigating those effects requires an understanding of the policy landscape of police decision-making and the basic psychology of suspicion. In this dissertation, I explore suspicion as a legal concept and as a psychological experience. I describe the role of the reasonable suspicion standard in judicial evaluations of the constitutionality of police practices, and the implications for the guidelines and trainings that agencies provide to officers. I contend that legal and quantitative analyses of policing practices should incorporate an understanding of the psychology of individual decision-making and the incentives created by the regulatory environment. The constitutional analysis assumes that civilian behavior, situational circumstances, and prior knowledge all affect an officer’s experience of suspicion and subsequent actions. Very little is known, however, about the basic psychology of suspicion and how it might affect judgment and decision-making. I investigate the psychological properties and covariates of interpersonal suspicion as reported by lay participants in a series of studies, establishing a baseline to which I will compare the effects of training and professional experience in future research. Using latent variable models and automated text analyses, I find that during experiences of interpersonal suspicion of a stranger, people tend to question the stranger’s intentions and experience intuition, attentiveness, and wariness. In these situations, distrust is more closely associated with emotional arousal than interpersonal suspicion. On average, female participants report slightly higher situational interpersonal suspicion relative to male participants, and participants who identify as Black or African American report lower suspicion relative to those who identify as White, Latino, or Hispanic. Relative to participants, the people who are targets of situational suspicion are more often described as male, Black, and Latino. On average, participants report a similar degree of suspicion across perceived target gender and racial categories, but there are significant differences among the associated emotions, inferences, and behavioral responses. Participants describing male and Black targets report experiencing greater fear and believing that the target’s behavior was dangerous. Participants describing male targets are more likely to report inferring that the target’s behavior was criminal, relative to participants describing female targets.

The dispositional tendency toward interpersonal suspicion is associated with neuroticism and low agreeableness in two samples of university students, and these findings are insensitive to variations in measurement instruments. In a simulation where university students take on the role of a police officer and report their suspicion in response to either Black or White male targets, I find that aggregate measures of dispositional interpersonal suspicion are uncorrelated with ratings of situational suspicion in response to the stimuli, which do not differ significantly by race of the target. An exploratory analysis suggests that dispositional suspicion, as measured by a single item, is associated with higher ratings of situational suspicion in response to White targets only.

My findings suggest that during experiences of interpersonal suspicion of strangers, people tend to question the stranger’s intentions and experience intuition, attentiveness, and wariness, and that the type of cognitive arousal associated with suspicion may be context-specific. In the concluding discussion, I also identify findings that could be particularly relevant in the legal context, including the salience of intuition in experiences of suspicion and the variation associated with target race in the correlates of suspicion. I aim to advance the current understanding of suspicion and establish a foundation for future research on its role in legal decision-making.