We pile on "to-dos" but don't consider "stop-doings." We create incentives for good behavior, but don't get rid of obstacles to it. We collect new-and-improved ideas, but don't prune the outdated ones. Every day, across challenges big and small, we neglect a basic way to make things better: we don't subtract. Leidy Klotz's pioneering research shows why. Whether we're building Lego® models or cities, grilled-cheese sandwiches or strategic plans, our minds tend to add before taking away. Even when we do think of it, subtraction can be harder to pull off because an array of biological, cultural, and economic forces push us towards more. But we have a choice — our blind spot need not go on taking its toll on our cities, our institutions, and our minds. By diagnosing our neglect of subtraction, we can treat it.