For more than a decade, my colleagues and I have been studying a form of invisibility known as inattentional blindness. In our best-known demonstration, we showed people a video and asked them to count how many times three basketball players wearing white shirts passed a ball. After about 30 seconds, a woman in a gorilla suit sauntered into the scene, faced the camera, thumped her chest and walked away. Half the viewers missed her. In fact, some people looked right at the gorilla and did not see it.

That video was an Internet sensation. So, in 2010, I decided to make a sequel. This time viewers were expecting the gorilla to make an appearance. And it did. But the viewers were so focused on watching for the gorilla that they overlooked other unexpected events, such as the curtain in the background changing color.

How could they miss something right before their eyes? This form of invisibility depends not on the limits of the eye, but on the limits of the mind. We consciously see only a small subset of our visual world, and when our attention is focused on one thing, we fail to notice other, unexpected things around us—including those we might want to see.

Consider, for instance, a famous 1995 incident in which police were in hot pursuit of four suspects driving away from the scene of a shooting. After cornering the suspects, the first police officer on the scene, Michael Cox, chased one of them on foot. Other officers arriving on the scene mistakenly thought Cox was a suspect and beat him. Meanwhile, another officer, Kenny Conley, had taken up pursuit of the same suspect and ran right past the altercation. Conley claimed not to have seen Cox or his assailants, and he was convicted of perjury and obstruction of justice.

Conley’s conviction raised an intriguing legal issue: Could an eyewitness actually fail to notice an assault like that one? Last year, psychology professor Christopher Chabris and I decided to put Conley’s alibi to the test. 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.

Indeed, most of us are unaware of the limits of our attention—and therein lies the real danger. For instance, 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.

Inattentional blindness does have an upside. Our ability to ignore distractions around us allows us to retain our focus. Just don’t expect your partner to be charitably disposed when your focus on the television renders her or him invisible.