The Formula That Changed How I Think About Design

A few years ago, I stumbled across a formula from Kurt Lewin, a psychologist working in the 1930s and 40s:

B = f(P, E)

Behavior is a function of the Person and the Environment.

Lewin was explaining why people do what they do. His answer: it’s never just who someone is or where they are. It’s always both, interacting. A person who’s calm at home might be anxious at work. Same person, different environment, different behavior.

All Design Is Behavior Design

The entire point of design is to get someone to do something. A poster wants you to attend a concert. A checkout flow wants you to complete your purchase. An onboarding sequence wants you to reach the moment where you understand why this product matters.

We don’t design artifacts. We design behaviors.

You Only Have Two Levers

Understand the person. This is user research—interviews, observation, usability testing. You can’t change the person, but you can understand them well enough to design for who they actually are.

Shape the environment. The app, the website, the physical space. You control this. Every button placement, every word choice, every default setting—these are environmental factors that interact with the person to produce behavior.

When a feature isn’t working, Lewin gives you a diagnostic. Is it a Person problem—did you misunderstand your users? Or an Environment problem—is there friction, confusing language, a buried button?

When someone says “users are stupid,” Lewin is your rebuttal. Users aren’t stupid. The environment you created is producing that behavior. Change the environment.

The Hard Problem: When People Are the Environment

Social computing breaks this model in uncomfortable ways. On a social network, other users become the environment. The content your user sees? Created by other people. The tone, the norms, the toxicity or kindness—all emergent from millions of individual actors you can’t control.

You can tweak the algorithm. You can write community guidelines. You can design reporting flows. But you’ve lost direct control of the E in Lewin’s equation. This is why social platforms are the hardest design challenge there is—and why so many of them are slowly eaten alive by the behavior they accidentally cultivated.

The Humbling Part

Lewin’s formula reminds you what you can’t do. You cannot redesign the person. You get the users you get—with their existing habits, their limited attention, their varying levels of motivation, their phones buzzing with notifications from seven other apps.

Your job is to understand those people deeply, then craft an environment that makes the behavior you want the natural, easy, obvious thing to do.

B = f(P, E). Memorize it. It’s the most useful psychology you’ll ever learn.