It was a summer evening when Tony Cornell tried to make the residents of Cambridge, England see a ghost. He got dressed up in a sheet and walked through a public park waving his arms about. Meanwhile his assistants observed the bystanders for any hint that they noticed something strange. No, this wasn’t Candid Camera. Cornell was a researcher interested in the paranormal. The idea was first to get people to notice the spectacle, and then see how they understood what their eyes were telling them. Would they see the apparition as a genuine ghost or as something more mundane, like a bloke in a bed sheet?

The plan was foiled when not a single bystander so much as raised an eye brow. Several cows did notice, however, and they followed Cornell on his ghostly rambles. Was it just a fluke, or did people “not want to see” the besheeted man, as Cornell concluded in his 1959 report?

Okay, that stunt was not a very good experiment, but twenty years later the eminent psychologist Ulric Neisser did a better job. He filmed a video of two teams of students passing a basketball back and forth, and superimposed another video of a girl with an umbrella walking right through the center of the screen. When he asked subjects in his study to count the number of times the ball was passed, an astonishing 79 percent failed to notice the girl with the umbrella. In the years since, hundreds of studies have backed up the idea that when attention is occupied with one thing, people often fail to notice other things right before their eyes.

When you first learn about these studies they seem deeply strange. Is it really possible that we are constantly failing to notice things right in front of us? Is there some mysterious force screening what we see and what remains hidden? According to Neisser the answer is yes, we are constantly overlooking much of the world around us and no, there is nothing mysterious about it. The key is to realize that this is just what attention is: selectivity. For a brain with finite computing power, zooming in to focus on one thing always means picking up less information about everything else. That’s how we are able to concentrate on anything at all and leave behind the blooming, buzzing bundle of distraction that is the rest of the world. It is also why being absorbed in a basketball game renders us blissfully oblivious to all requests to take out the garbage. Prioritizing one thing and neglecting everything else are two sides of the same coin.

Simple selectivity cannot be the end of the story, though, because recent research suggests that we miss some unattended things more than others. That’s right – the brain is selectively selective. In new research my colleagues Jazmin Brown-Iannuzzi, Sophie Trawalter, Kelly Hoffman and I pushed the idea of selective selectivity further by asking whether the unconscious screener might have priorities of its own. Scads of studies have suggested that the unconscious mind is riddled with stereotypes and biases, even among people who are consciously well intentioned. We asked whether the unconscious screener is prejudiced.

We started with a video of two teams passing basketballs around, as in Neisser’s early study. Then we superimposed a video of a young black man or a young white man walking across the screen. Would there be a racial disparity in which man gets noticed? We predicted that it would depend on the kind of goals the study’s participants had in mind. For decades, social scientists have known that prejudices show a social distance effect: people are more approving of stereotyped groups at a cold, impersonal distance than when they are up close and personal. For instance, polls show that whites are more likely to support equality for black Americans at a distance (such as saying that they support integrated neighborhoods and workplaces) than to support close personal ties (such as saying that they approve of someone in their family having an inter-racial marriage). Although attitudes toward all of these topics have become steadily less prejudiced since the 1960’s, the gap between close and far social distances has remained remarkably constant.

We told groups of white women that in a few minutes they would be asked to look through some online profiles of men to pick the best match for one of several roles. Two groups were to look for a socially distant match (a neighbor or a co-worker) and two would look for a more intimate match (a friend or a date). A separate control group was not told anything about searching profiles at all.

After they understood what they were going to look for, we interrupted the study to say that first they had to complete a concentration test to make sure they were paying attention. The concentration task was —you guessed it — the ball tossing video, and the participants were asked to keep their eyes on the ball. The real question was whether the women would be more likely to see the white man when they on the lookout for a close connection.

About two thirds of the women never saw the man walk across the screen in front of them, similar to the previous studies. As we suspected, who they saw depended on what they had on their minds. When the women were set to look for a suitable neighbor or co-worker they saw the black man and the white man equally often. But when they were looking for a friend or a date, they noticed the white man more than twice as often as the black man. The unconscious screener seemed to have racial preferences, but there was not a simple bias to see only whites or only blacks. The women were unconsciously deciding whether the man in the video was the kind of guy they were looking for. If not, he was never consciously perceived.

The simple fact of selectivity has big consequences: at any moment we are aware of just a tiny fragment of all that is around us. Consider your visual experience right now. There is usually no experience of a line in your periphery where your vision stops; there’s just a fading out of what you notice. You can move your eyes around to find the edges, of course, but you normally don’t notice the absence. If you look down, even your nose seems to get out of your way: a half-transparent thing that you almost never see without a mirror despite that it is in plain sight from your eye sockets.

Some psychologists and philosophers think that the rich and detailed conscious experience of the world around us is a grand illusion. The refrigerator light always appears to be on because when it is dark we aren’t looking. Just so, our conscious experience seems to be a rich and detailed picture of the world because where it’s not, we aren’t paying attention.

The idea of selective selectivity means that the unconscious mind may be shaping our experience even more dynamically than previously thought, screening what we see based on goals and emotions. Scientists are only starting to understand how selective selectivity works. Consider what happens when each of your eyes sees something different. That doesn’t happen in daily life, but in the laboratory scientists use a special kind of goggles that project images to each eye independently. One eye sees a face, say, and the other sees an elephant. Do you experience two pictures simultaneously, or a mixed up face with an elephant trunk? Neither: conscious experience toggles back and forth between a face one moment and an elephant the next. The unconscious screener is fickle, but decisive.

Psychologists Georg Alpers and Paul Pauli recently tested whether some kinds of pictures are more likely to be seen than others. On some trials of their experiment, one eye saw a neutral picture like a lamp, and the other eye saw a bloody scene of violence. On other trials, one eye saw a neutral picture and the other saw an erotic nude picture. The subjects’ conscious experiences of the pictures flipped back and forth, but the scenes of sex and violence were more likely to be the first ones seen, and they occupied consciousness much longer than the boring neutral images. Several studies have now confirmed that dangerous things like snakes, angry men, and snarling dogs can break through our concentration and intrude on consciousness. Dirty words and naughty pictures have the same effects. (I imagine it took a lot of delicate conversations between professors and the university ethics boards to produce this knowledge.) The common thread seems to be emotion. If it gets your heart racing it will get your attention.

This makes good evolutionary sense. It is important to be selective so that the mind can devote most of its resources to the task at hand. But it is also useful to keep an eye or an ear out for the unexpected (especially if it might eat you or you might mate with it). One minute you are sitting at a sidewalk café happily immersed in your newspaper and espresso. The whooshing traffic and the singing birds and the panting joggers all fade away as you lose yourself in the latest political battle between your favorite party and the irrational maniacs who disagree with them. And then a sexy jogger with a snarling pit bull saunters by. Who can concentrate on politics?

More to the point, who decided that this particular jogger gets to dominate your consciousness but the previous five were consigned to invisibility? It couldn’t be the conscious “you” who decided, because by the time you became aware of the jogger, the decision had already been made. There must be some part of the mind that is triaging the sights and sounds, but based on what, exactly?

Critics of this research suggest that it may be driven by something other than emotion. In one early study that flashed pictures of religious symbols to each eye, Catholic subjects were more likely to see a Crucifix and Jewish subjects were more likely to see a Star of David. Critics argued that this difference was not about personal significance, but simply the fact that Catholics had seen more Crucifixes and Jews had seen more Stars of David, which made them easier to process. Is there something about the red of blood or the anatomy of a nude body that that sets off unconscious alarms irrespective of emotional significance?

Psychologist Emily Balcetis and colleagues tackled this problem by holding the pictures constant and changing how much they meant to people. One group of subjects in the study was told that they could earn extra chances in a lottery for each letter they could identify in their goggles. Another group earned extra chances for each number. Pictures of letters and numbers were flashed to each eye so quickly that there was only time to see one or the other. When it paid to see a number, people saw a number. When it paid to see a letter, they saw a letter.

Scientists have argued for decades about how smart the screener is. Some believe it is a simpleton, able to detect basic sensory characteristics like light, color, and motion, but not able to read words for meaning or recognize what a picture is. If this view is right, it would be easy to break the screener down into simpler parts and understand it because it would be doing nothing more sophisticated than your digital camera. But the simpleton hypothesis cannot explain selective selectivity. It cannot explain why some events become visible or invisible based on what they mean to you. This trick requires a smarter screener.

The question is how smart does the screener need to be to explain these findings? No scientist today believes in a Freudian unconscious, complete with its own quirks and urges, scheming to delude the conscious mind. The unconscious today is understood as a vast store of knowledge, habits, and associations that help process information efficiently rather than waiting in the queue for slower conscious thinking. To explain selective selectivity, the unconscious screener must be able to do at least two things. First, it has to know what the goal is. Second, it must make a first approximation of whether the candidate for consciousness fits the goal or not.

This simple two-step comparison can explain why emotional events like dangerous and sexy things break through, because goals as basic as having sex and not being eaten are always relevant. It is not yet clear how sophisticated the screener can be. Our findings of racial bias, however, suggest something new about the assumptions the unconscious makes. At a minimum, our findings imply that the unconscious can represent social goals such as looking for a friend, a date, or a co-worker. And it seems to have opinions about which kind of people are suitable for each. These kinds of distinctions are more sophisticated, and perhaps more disturbing, than we had assumed.

There is something particularly disquieting about this brand of bias, because there is a power asymmetry. The unconscious screener shapes what the conscious “you” gets to see, but the conscious “you” doesn’t have veto power over that decision. Of course, you could try to shift your attention or change your goals once you are aware of them, but by then it may be too late. The unconscious always has a head start.

Personal contact between people of different races has always been seen as a powerful way to reduce prejudice. As the world becomes increasingly multicultural and globalized, these unconscious blinders might make us immune to that diversity. We cannot get to know or learn from people if we look right through them. The modern world might magnify these effects in a second way, because the power of the unconscious is greatest when our attention is under the heaviest demands. In today’s multitasking world, when we split our attention between Facebook and real friends, between our kindles and our kids, between our laptops and our loved ones, we delegate ever more to the unconscious. It makes you wonder who you have looked at today and have not seen.

Are you a scientist who specializes in neuroscience, cognitive science, or psychology? And have you read a recent peer-reviewed paper that you would like to write about? Please send suggestions to Mind Matters editor Gareth Cook, a Pulitzer prize-winning journalist and regular contributor to NewYorker.com. He can be reached at garethideas AT gmail.com or Twitter @garethideas.