Memory Constraints: UX and Human Psychology

This article is an excerpt from the Shortform book guide to "The Design of Everyday Things" by Don Norman. Shortform has the world's best summaries and analyses of books you should be reading.

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What are human memory constraints? Why should you take them into account when considering design?

Humans use their memories constantly to explore and process the world around them. Understanding memory constraints is an important part of design.

Read more about memory constraints and why they matter for design.

Memory Constraints

Memory constraints are conditions that limit the ways we can interact with an object or system. These conditions can be physical, cultural, semantic, or logical. (The four categories of constraints will be covered in depth in Chapter 4.) Norman uses the ancient practice of traveling performers reciting epic poetry as an example. Reciting even a third of an epic poem like Homer’s Odyssey or Illiad requires memorizing nine thousand lines, roughly equivalent to two and a half hours of recitation. This feat sounds impossible, in part because it is. Studies show that skilled performers are not reciting the poem word for word from memory, but actually recreating it on the spot using the constraints of poetry.

Depending on the form, poems have to follow certain rules of structure and rhyme. These rules act as constraints. If a performer tries to remember the last word of a line of poetry from the full English lexicon, they’re sunk. But if they know the word has to rhyme with the line before it, and it also has to convey a certain meaning, those memory constraints make it much easier to recall the right word.

Imagine trying to remember a certain word while reciting a poem. If you know from the rhyming structure that the word has to rhyme with “ear,” that narrows the field, but not by much: The word could be “fear,” “dear,” “sneer,” and so on. But if the word also has a constraint on its meaning, and has to mean “a unit of time,” the combined restraints allow for only one answer: “year.” With these constraints, a performer only needs to know the narrative of the story and the form of the poem to be able to produce what looks and sounds like an exact, word-for-word recitation.

Another example of this is taking apart an appliance to replace a part. Without memorizing, how do we know which parts go where when it comes time to reassemble? Physical constraints simplify this process: Certain size screws will only attach to certain size nuts, and parts of a particular shape will only slot into spaces that accommodate that shape. The physical nature of the objects themselves limits the ways they could be assembled, reducing the need to memorize the layout.

For designers, understanding memory constraints is crucial for creating a positive user experience. Designing objects and programs that are constrained by physical, cultural, or semantic rules takes the burden off the user, since the object itself limits the possible ways to interact with it.

Memory and Design

The subjective way memories are encoded affects how we retrieve them later on. Details of an event that were especially meaningful to us might be remembered as much more important to the overall story than they were at the time. In fact, whether information is meaningful is one of the biggest factors influencing our ability to remember it.

Meaningful things are easy to remember. They don’t need to be meaningful to our personal lives, as long as they have a meaningful relationship to each other, or to something else we know. Meaning helps us connect the information to a bigger picture. Arbitrary, unrelated things are much more difficult to remember. This is why rote learning is so difficult to do—the information being learned has no underlying structure to provide meaning. Typically, we cope with this by imposing structure of our own.

For example, the order of letters in the alphabet is arbitrary information. There is no underlying meaning explaining why C comes after B, except that we collectively agree that it does. To make this arbitrary sequence easier to learn, we impose structure in the form of the alphabet song.

Mnemonics are designed for this exact purpose. When learning to read music, remembering which notes correspond to which lines on the staff is arbitrary. Imposing the mnemonic “Every Good Boy Does Fine” turns an arbitrary series of notes (“EGBDF”) into a meaningful phrase, making it much easier to remember.

Another way we turn arbitrary information into meaningful information is through interpretation. The author gives an example of his friend, Professor Yutaka Sayeki, learning to use the turn signal on his motorcycle. The signal was mounted on the left handle bar, and signaled a left turn by pulling the lever backwards and a right turn by pushing forwards.

At first, Sayeki struggled to map the direction of the signal lever (forward or back) onto the direction of the signal light (left or right), since the connection between the two seemed completely arbitrary. To fix this, Sayeki adjusted his mental model of the turn signal so that the lever corresponded to the direction the handlebars moved when turning, as opposed to the direction of the motorcycle itself. Since pulling the left handlebar back turns the motorcycle to the left, and pushing the left handlebar forward turns the motorcycle to the right, this connection made sense—the information became meaningful and was then much easier to remember.

Designers can make this process much easier by creating meaningful controls as a way to work around memory constraints. For example, in a traditional car, the turn signal is pushed up to signal right and down to signal left. This takes advantage of our sense of clockwise and counter-clockwise direction: If we could extend the motion, the turn signal lever would be like the hand of a clock, and pushing up on it would ultimately send it to the right.

Memory Constraints: UX and Human Psychology