How Reliable and Robust Is Human Ability to Recognize Suspicious Activity?

Introduction
Security procedures at large public venues and transportation hubs rely upon vigilant and engaged security officers who are tasked, in part, with timely and appropriate responses to suspicious behaviors (behavior that seem unusual or out of place, that indicates that someone is in the process of planning or committing a malicious act) of potential hostiles (be they criminals, or terrorists) looking to victimize normal site users. This includes individuals conducting hostile reconnaissance, defined as “purposeful observation with the intention of collecting information to inform the planning of a hostile act against a specific target” (CPNI, 2016).

The presumption is that hostiles, armed with the ‘guilty knowledge’ of their true intention will behave or present in non-normative ways versus normal site users and thus provide opportunities for security to detect these suspicious behaviors (Gill et al., 2020). But how capable are individuals at detecting suspicious behavior? This systematic review assesses the current evidence base for the human ability to accurately recognize suspicious behavior. 

The evidence for a narrower form of deception – lie detection – paints an interesting picture. In terms of lie detection, Bond and DePaulo’s (2006) meta-analysis found that just 54% of untrained observer judgments were correct, only slightly higher than chance. Performance was worse when observers could only see the target person (52% accuracy), than when they could only hear them (63%). However, liars are more nervous and more conscious of their own behavior than truth tellers (Vrij, 2008; Vrij et al, 2019) and when being interviewed are aware that they are being actively observed and scrutinized. Those with hostile intent may not believe that they are being watched, but they may be vulnerable to the spotlight effect – a tendency to believe they are being noticed more than they are and as such overestimate the extent to which they are the focus of the attention of others (Gilovich et al, 2000).

Conclusions
Individuals differ in cognitive and perceptual skills and therefore infer different meanings from viewed behavior. These differences in the interpretation of cues may affect the ability to accurately detect suspicious behavior. Observers of the environment need knowledge of behaviors linked to hostile intent, however establishing non-verbal indicators of hostile intent that are accurate across many contexts is difficult. Cues of intent may not be expressed in cases where the crime is expressive or spontaneous. Where they are apparent, they may be difficult to interpret accurately due to the observer’s absence of the perpetrators baseline ‘normal’ behavior with which to compare. As these behaviors may deviate from situationally appropriate conduct observers also need knowledge of ‘normal’ behavior for that specific location.

Of course, offenders can deliberately modify their behavior to conceal intent. There may be overlap between normal and suspicious behavior in the same situation. As well as difficulties in establishing a universal baseline of behavior that is applicable in every context, problems also arise in keeping natural guardians vigilant. Tasking members of the public to perceive a scene as a whole, and then try to detect clusters of behavior that differ from the baseline is not feasible. Security system operators may not have an increased ability to identify suspicious behaviors except for when they have an understanding of the norms of the given environment. Little is known about the strategies observers of CCTV use when monitoring and interpreting behavior.

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