Serendipitously I came upon the same psychological demonstration twice in a ten-day span. It appears in The Tipping Point by Malcolm Gladwell and is discussed by Paul Bloom in Yale’s intro psych course (which I reviewed). If you haven’t read Gladwell’s book or taken an intro psych course perhaps you have not seen this demonstration. It illustrates the powerful role of context in human cognition. The demo follows.

I will present the same puzzle twice. After you read the first version, write down your first, intuitive answer without thinking it over a great deal. Only then go on to the second version and do the same. Don’t cheat and read ahead or you’ll ruin the whole exercise for yourself. It is more rewarding to do this as instructed.

Version 1. Each of the cards shown below have a letter, “B” or “M”, on one side and a positive integer on the other. As shown below, you can only see one side of each card. The cards are alleged to have the property that any card with a “B” on one side has a number greater than or equal to 21 on the other. Which cards must be turned over to verify this property.

Remember, don’t think too long. Just write down your hunch using your intuition. If you’ve written down your answer go on to version 2.

Version 2. You’re a bartender serving only milk and beer. The cards shown above correspond to four people at your bar (each card represents one person). Letters correspond to what they’re drinking and numbers to their ages. “B” stands for “beer” and “M” stand for “milk.” The minimum legal drinking age is 21. Anyone drinking beer (“B”) must be 21 years old or older. Which cards must be turned over to verify compliance with the law?

The correct answer in both cases is that you need to flip the card marked “B” and the one with the number 20. Why? Because you have to verify that every card with a “B” (a person drinking beer) has a number (age) of at least 21. So you have to check that the card marked “B” (person drinking beer) has a number (age) on the other side no smaller than 21. Also, you have to check that the card (person) with the number (age) 20 does not have a “B” (is not drinking beer).

There is no point in flipping the card marked “M” because there is no rule (law) pertaining to “M”. It doesn’t matter what number is on the back of the “M” card. There is no need to flip the card with the number 25 because it doesn’t matter what letter (drink) is associated with it. It would not be out of compliance with the rule (law) in either case.

Did you get the answer right after reading version 1? What about after reading version 2? More people get version 1 wrong than version 2. That is, many people who find version 1 intuitively difficult find version 2 much easier. Why?

The answer is context. The only difference between the two versions is the story told. The raw data and task are the same. This illustrates that context is extremely important for problem solving. At least in the U.S., we’re accustomed to minimum drinking age laws. When the puzzle is described in that context it is easier to comprehend. Our mind manipulates the familiar ideas more easily even though it is the same manipulation required in both versions. That’s interesting.

What is also interesting is that this property of cognition–that context matters–is not always obvious to us. That’s typical of many of the ways in which our perception and thoughts are shaped by the nature of our minds. We are usually not aware of how or why we perceive and think what we do. We go about our day with the notion that we’re not missing anything important, that our arguments are free of bias, that we are masters of our reason. Context is just one of many elements of our environment and background that shape our perception and thought. Take a problem out of a familiar context and it becomes much harder. We’re more likely to get it wrong.

In fact, we’re wrong more often than we think we are, but we don’t notice. Only in the context of a psychological demonstration is the power of context clearly revealed.