Why an Invisible Gorilla Is a Security Threat

On “inattentional blindness” in security guards, and what to do about it.
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On “inattentional blindness” in security guards, and what to do about it.


You may have seen a video online somewhere, or in a Psych 101 class perhaps, of a group of people wearing black and white shirts passing a basketball back and forth. When you watch, you are prompted to count how many times the players wearing white shirts pass the ball. What you may not see, though, even though it is right in front of you, is that after a half a minute of the basketball-passing and feet-shuffling, someone wearing a gorilla suit strolls into the center of the screen, thumps her chest, and then walks away.

Many people are so engaged with the assignment of counting ball-passes that they don’t notice the gorilla at all; the first clue they have is the video’s gleeful reveal: “How many passes did you count? The correct number is 15 passes. But did you see the gorilla?!” This video was originally used by psychologists Daniel Simons and Christopher Chabris, in an experiment testing the curious phenomena of “inattentional blindness.” Simons has described this as “a form of invisibility” that “depends not on the limits of the eye, but on the limits of the mind.”

Outside of fake-gorilla-spotting, this ability of our eyes and brains to focus on one thing in a world of distractions is very valuable—life-saving, even. But our very lack of awareness of these limitations can also have very dangerous consequences. For instance, Simons mentions the false confidence involved in using a cell phone while driving: “We may talk on the phone and drive because we are mistakenly convinced that we would notice a sudden event, such as a car stopping short in front of us.”

Simons also wrote about how inattentional blindness played a role in a 1995 conviction of a Boston police officer, Kevin Conley, for perjury. The officer had claimed in court that, while chasing a suspect from the scene of a crime, he had run right past a huge fight involving another officer. In an experiment inspired by this court case, Simons and Chabris tested Conley’s alibi and denial. As Simons later described:

Although we could not simulate a high-speed police pursuit, we could extract the most critical element: Conley’s focus on pursuing a suspect. In our experiment, we asked participants to jog behind an assistant and count the number of times he touched his hat. As they jogged, they ran past a staged fight in which two men appeared to be beating a third. Even in broad daylight, over 40 percent missed the fight. At night, 65 percent missed it. In light of such data, Conley’s statement that he didn’t even see Cox or his assailants was plausible.

Now, a new Carnegie Mellon study addresses a realm in which inattentional blindness presents another particular set of problems: security cameras. In a series of experiments simulating video surveillance, researchers demonstrated how easily people may miss blatant security threats when they are focused on the wrong thing. The participants watched simulated security-camera footage for two hours, during which they were told to count and categorize the hats that people were wearing. They were also told to click a button if they saw a security threat anywhere. The results were similar to the invisible gorilla test, if a little more dark: “Most missed events were blatant, such as someone brandishing a knife,” according to a release about the study.

Judith Gelertner, one of the authors, explained:

This is a problem not just for airport screeners, border guards or building security personnel, but for any occupation in which someone is looking for anomalies that occur infrequently, such as radiologists reading CT scans, or pathologists looking for cancer in biopsy slides. [It] occurs because the brain must strike a balance between accuracy, which can take time, and efficiency. When a person has seen a lot of a certain pattern, such as law-abiding behavior, the brain might continue to register that pattern, despite what is seen to the contrary, in an attempt to be efficient, she said.

Gelertner also suggested a potential solution to this problem: repetition, and practice. Experiment participants were much more likely to detect all of the threats when there were more of them; the repetition kept them alert. Security guards may watch their video screens forever without seeing anything resembling a threat. But perhaps repeated, simulated threats would help them see what to look for, keep them alert, and gradually teach their brains not to filter out the rare moment of danger when and if it arrives.