Since Cooper invented personas, they've become the de facto interaction design tool across the world. Most folks know the basics of why: They embody salient user characteristics from first-hand user research in a medium that people are good at understanding, remembering, and discussing, i.e. people.
I've been using personas as a design, teaching, and even writing tool for over a decade now and I believe strongly that all that's true. They do embody research. They are easy to remember and use. But I think there's another reason they work, a psychological reason, that's as important to understanding them: they get you to think about design problems in a fundamentally different way.
This effect is not just true for interaction designers, either. Whether you're a developer, product owner, business strategist, or content strategist, the reason they work has to do with the way that you think about the world around you. To explain why, we need to tuck in to a little philosophical thinking. Don't worry, it will be quick and painless.
The philosopher Daniel Dennett proposed that the way people reason about things the world around them changes based on the particular one of three stances they take toward those things.
When we adopt a physical stance, we use our basic sense of physics to predict things, such as when we wonder, “What would happen if I pour sand on a fire?”
When we adopt a design stance, we use our understanding of human intentions as they are embodied in designed objects, such as when we pick up a new tool and wonder, “How did the person who made this intend for it to be used?”
When we adopt an intentional stance, we use a different part of our brain to anticipate how an intentional agent (like a plant, animal, or person) will change their behavior in order to get what he, she, or it wants. When we predict what the tiger that's chasing us will do when we turn to duck in a cave, we're adopting an intentional stance.
When we design, we are trying to optimize systems towards some desired effect. This means that we're taking one of these predictive stances about what will and won't work, i.e., what will let our personas achieve their goals. Which stance is it? I don't think anyone adopts the physical stance when designing. No one perceives users' behaviors as defined by strictly physical forces. But what of the other two stances?
I believe the design stance is the default stance people take when designing, (apologies for the redundant-sounding phrasing) and it's the wrong one. In the design stance you can change the tool to fit the task. Turn the hammer around. Pull off the metal part. Only instead of working with hammers, you're working with your sense of users in the world. This gives rise to problems of “the elastic user” discussed in The Inmates, which is to say that we can cherry-pick whatever aspects of users suit our interests at hand. When we design this way, we serve ourselves and the systems more than we do the people who are using our stuff, and that's a way to ensure that people do not love your product or service.
The intentional stance is the right stance for design, since it respects users’ goals as something largely fixed, and which we have to accommodate with the smartest design possible, or they will change their behavior, and go to a competitor.
Even if this were just philosophy it would make sense, but it turns out that a number of universities including Glasgow Caledonian University and MIT have done some experiments that show that in fact, we do use different parts of our brains in each stance. They've even captured it in functional images. It’s not just philosophy and common sense, it’s actual neuroscience. (For an academic reference, see below.)
Once you understand this "other" reason personas work, a lot of the little nuanced guidelines that I try and instill in young interaction designers and students make sense. Why don't we want our personas to have names like "Cathy Consumer" and "Adam Analyst"? Because no one in the real world would have a name like that. It's is clear that these are fabricated objects rather than real people. Such names get your teams into a design stance where the facts of actual users can be skewed or ignored. Rather, it's best to choose names that could be in the real world but aren't in and of themselves distracting or hard to pronounce. Persona names could belong to a real person, which helps encourages an intentional stance.
For similar reasons, it's why we give personas big photos that don't look like models. It's why we provide some narrative background to them, including education, training, coworkers, or family as the project suggests. It's why we avoid highlighting the data that went into them on their introductory slides. It's why we really only mention that we created them once, and thereafter refer to them as real people. We want them to trigger the sense of living, breathing intentionality in everyone who has a responsibility for accommodating those goals. This gives us the stance that will keep user goals foremost and consistently considered while possible design decisions fare the treacherous waters of the design and development process.
This is the other reason personas just work.
Gallagher, Helen L., and Christopher D. Frith. "Functional Imaging of ‘theory of Mind’." Learning Development and Resource Center. Trends in Cognitive Sciences, Feb. 2003. Web. 16 July 2012.