In a simulation of airport luggage scanning, a team of researchers has found that the rarer an item is, the less likely a scanner operator is to spot it—that is, if fewer people come through with bomb materials or guns, it will be harder for the operator to spot them when they do.

The Duke University scientists set up the simulation in an “Airport Scanner” app where participants would check virtual suitcases for a set of 78 verboten items, like a stick of dynamite or a gun. Thirty of the items were “ultra rare,” appearing less than 0.15 percent of the time.

Drawing upon 20 million searches, the team found that these ultra-rare items were more difficult for participants to spot than more common things. The ultra-rare items were spotted only 27 percent of the time, while items that cropped up in one percent of suitcases were correctly spotted 92 percent of the time.

The paper calls this the “ultra-rare item effect,” wherein “visual search for exceedingly rare items is highly susceptible to error.” For airport security, in theory, this means the more offbeat the weapon or destructive device of choice, the better chance the carrier has of clearing security.

Low frequencies of visual cues have been linked to an increase in error before, again in a rather important area: cancer screenings. While the players of the Airport Scanner game likely lacked the training that TSA agents receive for this type of job, the authors of the paper believe that the data could be helpful for the TSA’s training procedures.

Psychological Science, 2013. DOI: 10.1177/0956797613504221 (About DOIs).