How experts recall chess positions

In 2011, a computer (Watson) outplayed two human Jeopardy champions.  In 1997, chess computer Deep Blue defeated chess champion Garry Kasparov. In both cases, the computer “solved” the game—found the right questions or good moves—differently than humans do.  Defeating humans in these domains took years of research and programming by teams of engineers, but only with huge advantages in speed, efficiency, memory, and precision could computers compete with much more limited humans.

What allows human experts to match wits with custom-designed computers equipped with tremendous processing power?  Chess players have a limited ability to evaluate all of the possible moves, the responses to those moves, the responses to the responses, etc. Even if they could evaluate all of the possible alternatives several moves deep, they still would need to remember which moves they had evaluated, which ones led to the best outcomes, and so on.  Computers expend no effort remembering possibilities that they had already rejected or revisiting options that proved unfruitful.

This question, how do chess experts evaluate positions to find the best move, has been studied for decades, dating back to the groundbreaking work of Adriaan de Groot and later to work by William Chase and Herbert Simon.  de Groot interviewed several chess players as they evaluated positions, and he argued that experts and weaker players tended to “look” about the same number of moves ahead and to evaluate similar numbers of moves with roughly similar speed.  The relatively small differences between experts and novices suggested that their advantages came not from brute force calculation ability but from something else: knowledge.  According to De Groot, the core of chess expertise is the ability to recognize huge number of chess positions (or parts of positions) and to derive moves from them.  In short, their greater efficiency came not from evaluating more outcomes, but from considering only the better options. [Note: Some of the details of de Groot’s claims, which he made before the appropriate statistical tests were in widespread use, did not hold up to later scrutiny—experts do consider somewhat more options, look a bit deeper, and process positions faster than less expert players (Holding, 1992). But de Groot was right about the limited nature of expert search and the importance of knowledge and pattern recognition in expert performance.]

In de Groot’s most famous demonstration, he showed several players images of chess positions for a few seconds and asked the players to reconstruct the positions from memory.  The experts made relatively few mistakes even though they had seen the position only briefly.  Years later, Chase and Simon replicated de Groot’s finding with another expert (a master-level player) as well as an amateur and a novice.  They also added a critical control: The players viewed both real chess positions and scrambled chess positions (that included pieces in implausible and even impossible locations). The expert excelled with the real positions, but performed no better than the amateur and novice for the scrambled positions (later studies showed that experts can perform slightly better than novices for random positions too if given enough time; Gobet & Simon, 1996).  The expert advantage apparently comes from familiarity with real chess positions, something that allows more efficient encoding or retrieval of the positions.

Chase and Simon recorded their expert performing the chess reconstruction task, and found that he placed the pieces on the board in spatially contiguous chunks, with pauses of a couple seconds after he reproduced each chunk.  This finding has become part of the canon of cognitive psychology: People can increase their working memory capacity by grouping together otherwise discrete pieces of items to form a larger unit in memory.  In that way, we can encode more information into the same limited number of memory slots.

They In 1998, Chris Chabris and I invited two-time US Champion and International Grandmaster Patrick Wolff (a friend of Chris’s) to the lab and asked him to do the chess position reconstruction task. Wolff viewed each position (on a printed index card) for five seconds and then immediately reconstructed it on a chess board.  After he was satisfied with his work, we gave him the next card.  After he finished five real positions and five scrambled positions, we asked him to describe how he did the task.

The video below shows his performance and his explanations (Chris is the one handing him the cards and holding the stopwatch—I was behind the camera). Like other experts who have been tested, Wolff rarely made mistakes in reconstructing positions, and when he did, the errors were trivial—they did not alter the fundamental meaning or structure of the position. Watch for the interesting comments at the end when Wolff describes why he was focused on some aspects of a position but not others.

HT to Chris Chabris for comments on a draft of this post

Comments: Please make your comments on the Google+ notice of this post. That will permit more interaction:

Sources cited:

For an extended discussion of chess expertise and the nature of expert memory, see Christopher Chabris’s dissertation:  Chabris, C. F. (1999).  Cognitive and neuropsychological mechanisms of expertise: Studies with chess masters.  Doctoral Dissertation, Harvard University.

Chase, W. G., & Simon, H. A. (1973).  Perception in chess.  Cognitive Psychology, 4, 55-81.

de Groot, A.D. (1946). Het denken van de schaker. [The thought of the chess player.] Amsterdam: North-Holland. (Updated translation published as Thought and choice in chess, Mouton, The Hague, 1965; corrected second edition published in 1978.)

Holding, D.H. (1992). Theories of chess skill. Psychological Research, 54(1), 10–16.

Gobet, F., & Simon, H.A. (1996a). Recall of rapidly presented random chess positions is a function of skill. Psychonomic Bulletin & Review, 3(2), 159–163.

New guide to writing and revising

Over the past 20 years of teaching, writing, and editing, I have compiled a set of tips, tricks, and pet peeves that I share with students and colleagues. I’ve decided to make this writing guide more widely available in case others will find it useful. The emphasis is on scientific writing, but the same principles apply to most non-fiction (including journalism). The most recent version of the file is available from my personal website at

The first part of the guide gives some broad principles of effective writing. The next section provides suggestions for editing and revising a manuscript. After that comes a list of common writing mistakes and my pet peeves. The last section provides a revision worksheet. The worksheet is perhaps the most useful part of the writing guide. It is a systematic way to edit papers, progressing from high-level organization to the word level, with a box to check after you complete each step. By following the steps in the revision worksheet, your writing will be more concise, more precise, and easier to read.

If you use this guide or the worksheet for one of your own revisions, I’d be curious to hear whether it helped you. If you have suggestions for future versions of this file, I’d love to see those too. Rather than posting comments here, please post them to the following page on Google+:

Feel free to share the file itself with your students or fellow writers.

Ghost busters, parapsychology, and the first study of inattentional blindness

Originally posted last year our now-inactive Psychology Today blog:

Until last week, I thought I knew the full history of research on inattentional blindness. Inattentional blindness is the failure to notice a fully-visible but unexpected object or event when you are focusing attention on something else. I’ve been conducting research on the topic since the late 1990s, and I thought I was familiar with all of the work that came before mine. I knew all about Ulric Neisser’s work in the 1970s on selective looking, including many of his unpublished studies from that era. I knew about the dichotic listening studies that partially motivated his research. I knew about Arien Mack and Irv Rock’s groundbreaking studies during the 1990s on inattentional blindness in simplified computer displays (their book gave the phenomenon its scientific name). I knew about studies of tunnel vision in pilots as well as the literature on focused attention and distraction that provides a mechanistic explanation for what we see and what we miss.

Yet, none of those literatures cite what might well be the first experimental documentation of inattentional blindness. In fact, it’s likely that none of those researchers knew about these studies. None of us can really be faulted for missing them- they appeared in an unexpected place that fell well outside the focus of our research. The source is unlikely, but seasonally appropriate…

Last week I got an email from a colleague in our department about Mary Roach’s book “Spook: Science Tackles the Afterlife.” I haven’t read the book (although I’ve been meaning to-a couple of her other books are on my reading list now). In it, Roach cites a couple of studies (on pages 251-252) that addressed a somewhat bizarre question given modern scientific sensibilities: Is it possible to induce a “genuine” paranormal experience? Specifically, is it possible to make people believe they’ve seen a ghost?

More than 50 years ago, Tony Cornell, a parapsychology researcher, decided to test how people would react upon seeing him dressed as a ghost. Would they experience him as a “real” ghost or as something more mundane? He published a series of studies in two papers in the Journal of the Society for Psychical Research, the first of which was titled “An experiment in apparitional observation and findings.” He stated his goal:

An experiment in apparitional observation has been undertaken in Cambridge to determine how many people would claim to have experienced the seeing of an apparition or ghost.

Each night, Cornell or his assistants dressed in a white sheet and strolled down a path, making various hand gestures before shedding the sheet 4.5 minutes later. Other assistants observed how many people were “in a position to observe the apparition.” His finding: “although it was estimated that some 70-80 persons were in a position to observe the apparition, not one was seen to give it a second glance or to react in any way.” That’s true even though a number of cows apparently followed the ghost around.

Although Cornell’s finding is consistent with later studies of inattentional blindness, his conclusion isn’t. He finds it unlikely that nobody saw the ghost because:

a white-clad figure in the middle of a damp grass field, followed by a number of cows, is hardly to be ignored at the best of times…. If no one saw it consciously, one can only surmise that they did not want to see it. Cornell attributes the failure to notice the ghost to “the absence of a more subtle psi factor which is always present in genuine apparitional experience.

Cornell’s second paper, “Further Experiments in Apparitional Observation,” is even more prescient. He dressed as a ghost at a local movie theater. During a trailer film, he walked across the stage and back again, remaining visible for a total of 50 seconds. After the trailer, he polled the audience, and of those who responded, 68% claimed to have seen something and 32% claimed to have seen nothing peculiar, with many people not responding. Only about 50% saw anything during the ghost’s first pass across the stage, and many failed to describe it accurately.

Although you can feel Cornell’s disappointment that his “apparition” was not mistaken for a “real” ghost, his studies are remarkable both because they anticipated contemporary studies of inattentional blindness and because they actually tested inattentional blindness in the real world rather than in video or on a computer display. Only one other study has done that in the 50 years since (Ira Hyman and colleagues’ unicycling clown study from earlier this year). Perhaps more importantly, Cornell’s studies were the first to show how easily we can miss the fake paranormal events around us!

Hat tip to Carol Nickerson for spotting these citations in Roach’s book.

Sources Cited:

Cornell, A. D. (1959). An experiment in apparitional observation and findings Journal of the Society for Psychical Research, 40 (701), 120-124

Cornell, A. D. (1959). Further experiments in apparitional observation Journal of the Society for Psychical Research, 40 (706), 409-418

Hyman, I., Boss, S., Wise, B., McKenzie, K., & Caggiano, J. (2009). Did you see the unicycling clown? Inattentional blindness while walking and talking on a cell phone Applied Cognitive Psychology, 24 (5), 597-607 DOI: 10.1002/acp.1638

Think video games make you smarter? Not so fast...

Updated on 9/13/11 at 4:50pmMinor tone/wording update to the conclusion and a little more detail on alternative explanations for correlational results after the break.

Try to spot the flaw in this study. A scientist recruits a group of subjects to test the effectiveness of a new drug that purportedly improves attention. After giving subjects a pre-test to measure their attention, the experimenter tells the subjects all about the exciting new pill, and after they take the pill, the experimenter re-tests their attention. The subjects show significantly better performance the second time they’re tested.

This study would never pass muster in the peer review process—the flaws are too glaring. First, the subjects are not blind to the hypothesis—the experimenter told them about the exiting new drug—so they could be motivated to try harder the second time they take the test. The experimenter isn’t blind to the hypothesis either, so they might influence subject performance as well. There’s also no placebo control condition to account for the typical improvement people make when performing a task for the second time. In fact, this study lacks all of the gold-standard controls needed in a clinical intervention.

Walter Boot, Daniel Blakely and I have a new paper that just appeared in Frontiers in Psychology this week (link) that argues for similarly serious flaws in many of the studies underlying the popular notion that playing action video games enhances cognitive abilities. The flaws are sometimes more subtle, but they’re remarkably common: None of the existing studies include all the gold-standard controls necessary to draw a firm conclusion about the benefits of gaming on cognition. When coupled with publication biases that exclude failures to replicate from the published literature, these flaws raise doubts about the mere existence of a benefit.

Continue reading Think video games make you smarter? Not so fast…

Harry Potter and the Illusion of Potential

A variant of this post first appeared on my Psychology Today blog on November 16, 2010.

Why is the story of Harry Potter so appealing? The success of the series depends on engaging characters and compelling storytelling-it’s a classic tale of good vs. evil and a coming of age story. That’s true, but many stories have those qualities. I think there’s a deeper magic at work here, one that capitalizes on a pervasive cognitive illusion. It’s a cognitive illusion that underlies almost all fantasy (and much science fiction) writing and that contributes to the success of countless movies and television shows. It involves a sort of wish fulfillment.

As a child, I fantasized about how my life would change if I suddenly discovered my “Spiderman” powers and could scale buildings. Or, I envisioned how radically my life would change if I could figure out how to teleport myself instantly from one location to another (most of that fantasizing emerged when I was trudging home from school). Perhaps you have had similar fantasies, or maybe yours were more mundane: imagining discovering you had tremendous athletic prowess at a sport you had never tried or that you would be a virtuoso musician if you just found the right instrument.

We all, at times, fall prey to the illusion of potential-the belief that we can acquire skills or abilities with minimal effort or practice. The illusion of potential relies on the corollary belief that we have vast pools of untapped brain power just waiting to be released. The myth that we only use 10% of our brain is a direct statement of this idea. Hucksters use the belief in untapped potential to sell everything from miracle exercise regimens (great results with minimal effort) to ultra-fast speed reading. Self-claimed psychics argue that they discovered their abilities. Mentalists and magicians know that their audiences are likely to find appeals to untapped potential compelling and use them liberally in their patter.

Not surprisingly, popular culture gives people what they want. The idea of untapped potential is a staple of fantasy books and movies. In fact, it might well be the defining feature of classic fantasy writing; the central character discovers a hidden ability they didn’t know they had. One of the common features of science fiction writing involves changing one element of how the universe works and then playing out the consequences. In many cases, that defining characteristic involves untapped potential (e.g., Verner Vinge’s brilliantly conceived idea of “Focus” in his award-winning novels A Fire Upon The Deep and A Deepness In The Sky).

The theme of untapped potential pervades television dramas. The subtitle of NBC’s hit series Heroes actually restates the definition of untapped potential: “ordinary people discovering extraordinary abilities.” The success of Heroes inspired a slew of shows with the same theme: CBS’s The Mentalist features a detective with “unusual powers of observation,” ABC’s Section 8 was about “especially brainy individuals,” and Fox’s Lie to Me centers on “a man who uses his preternatural skill at reading body language to help solve mysteries.” (Note that Lie to Me is based loosely on Paul Ekman, a prominent psychologist and expert on face perception. If Ekman has exceptional skill in reading faces, it’s because he spent decades studying and training, not because he had some secret talent.) The “untapped potential” plotline is one of the oldest forms of narrative, the rags-to-riches story in which a character finds themselves suddenly transformed, revealing the princess hidden in the lowly servant.

The illusion of potential and the fantasy of discovering hidden powers helps explain the exceptional popularity of Harry Potter. In the books, some people have magical abilities waiting to be revealed and other people are “muggles.” Yes, they hone those skills, but the abilities are there waiting to be discovered and released. That one element-the discovery of a previously unknown ability that reveals itself with little effort or work-is central to the story’s success. It taps our fantasies and cognitive illusions. The Harry Potter stories allow us to vicariously experience the ability to teleport ourselves home from school. It’s cognitive illusion wish fulfillment at its best.

Gorilla suit winner

Last month, in celebration of the release of the paperback edition of The Invisible Gorilla, we teamed up with to raffle away a deluxe gorilla suit to one lucky buyer of our book. I have just been informed that the lucky winner is Professor Bennett Schwartz, a cognitive psychology professor at Florida International University. Even better, he plans to use the gorilla suit in his cognitive psychology class! (Just a heads up, Bennett — once you’ve worn a gorilla suit in class, there’s no going back…)

Congrats Bennett, and thanks to a href=””> and to everyone who participated in the promotion for making the launch of our paperback edition a success. The book is now officially a New York Times Bestseller!

The consequences of being ignored

I have just returned from the annual Vision Sciences Society meeting and saw some really fascinating presentations. Over the next couple weeks, I’ll feature a few of them. The first one addresses the consequences of being ignored (if you are a colored shape, that is).
Continue reading The consequences of being ignored (for colored shapes, at least)

The Best Illusions of the Year

On Monday evening I attended the Best Illusion of the Year Contest in Naples Florida. (I live tweeted it as well — that was far more of a challenge than I had expected. See @profsimons if you’re interested). The winning illusions always receive a lot of visibility online, so I thought I would draw your attention to two illusions that were not among the winners, but that were fantastic nonetheless. First, a little background.
Continue reading The Best Illusions of the Year

Another early study of Inattentional Blindness

Back in October on our Psychology Today blog, I posted about my re-discovery of what I then thought was the earliest systematic study of inattentional blindness. Turns out I was wrong.

Inattentional blindness is the failure to notice a fully-visible but unexpected object when you are focusing attention on something else. It is the phenomenon illustrated by our invisible gorilla studies. That study was conducted in the 1950s by a Tony Cornell, a parapsychology researcher — he found that people often didn’t notice him while he pranced around campus dressed as a ghost! (check out the post — it is one of my favorites. I’ll eventually repost it here).

As it turns out, there was at least one earlier experiment. Fifty years earlier, in fact. Just this week, my colleague Ira Hyman (he of unicycling clown fame) pointed me to a section of a chapter from a book by Hugo Munsterberg, written in 1908!

Image of Hugo Munsterberg's book

Hugo Munsterberg's book on eyewitness psychology. Source:

In it, he describes the following experimental result:

I stood on a platform behind a low desk and begged the men to watch and to describe everything which I was going to do from one given signal to another. As soon as the signal was given, I lifted with my right hand a little revolving wheel with a colour-disk and made it run and change its color, and all the time, while I kept the little instrument at the height of my head, I turned my eyes eagerly toward it. While this was going on, up to the closing signal, I took with my left hand, at first, a pencil from my vest-pocket and wrote something at the desk; then I took my watch out and laid it on the table; then I took a silver cigarette-box from my pocket, opened it, took a cigarette out of it, closed it with a loud click, and returned it to my pocket; and then came the ending signal. The results showed that eighteen of the hundred had not noticed anything of all that I was doing with my left hand. Pencil and watch and cigarettes had simply not existed for them.

That’s a pretty clear example of inattentional blindness, with at least some failure to notice objects and events happening outside the focus of attention. It has the flavor of a magician’s misdirection, with social cues directed to the other hand (see Kuhn, 2009 for an example).

My favorite part of Munsterberg’s report, though, was his description of his own mistaken intuitions. He too suffered from an illusion of attention, the belief that people typically notice salient and distinctive events.

I had made my movements of the left are so ostentatiously, and I had beforehand so earnestly insisted that they ought to watch every single movement, that I hardly expected to make any one overlook the larger part of my actions.

Just as we had expected people to notice when a person in a gorilla suit walked through a scene and thumped her chest at the camera, Munsterberg also had the wrong intuition about what people would and wouldn’t notice.

I might try to replicate his method in my own lab — it’s one of the few real-world studies of inattentional blindness and provides a nice intermediary to magical misdirection and laboratory studies of focused attention and inattention.

Sources Cited:

Munsterberg, Hugo (1908). On the Witness Stand: Essays on Psychology and Crime. Clark Boardman Co., NY, New York.

Munsterberg’s book is available online here.

Kuhn, G., Tatler, B., & Cole, G. (2009). You look where I look! Effect of gaze cues on overt and covert attention in misdirection Visual Cognition, 17 (6), 925-944 DOI: 10.1080/13506280902826775

Hyman, I., Boss, S., Wise, B., McKenzie, K., & Caggiano, J. (2009). Did you see the unicycling clown? Inattentional blindness while walking and talking on a cell phone Applied Cognitive Psychology, 24 (5), 597-607 DOI: 10.1002/acp.1638

Cornell, A. D. (1959). An experiment in apparitional observation and findings. Journal of the Society for Psychical Research, 40(701), 120-124.

invisible gorillas and motorcycle safety

One of the most common sorts of motorcycle accidents involves a failure of attention: The driver of a car turns left in front of an oncoming motorcycle, failing to yield the right of way. In many cases, both the rider and driver report that the driver looked right at the motorcycle before turning—they looked without seeing. Such accidents could well be an example of the illusion of attention, our tendency to assume that we will notice anything important that happens right in front of our eyes.

For the launch of our paperback edition, we are conducting a charity promotion, and one of the participating charities is the BMW MOA Foundation, a rider group that actively promotes safety education through their magazine and Rider Performance University. I recently penned a short essay for the magazine and website on how everyday illusions and mistaken intuitions increase the danger for motorcyclists. You can read it here.

If you’re a rider (or a bicyclist) and have experienced a looked-but-failed-to-see accident, please share your experiences with readers in the comments. (And, if you pre-order our paperback, you can target our donation to the BMW MOA Foundation or to any of a number of other great charities).