When figuring out how to tackle a problem, our instincts are almost always to add: we make to-do, not to-don’t lists after all. But just because humans have a harder time seeing subtraction — which can come in the form of tearing down buildings, dismantling barriers, and pruning old ideas — as a viable solution doesn’t make it any less useful of an approach. Leidy Klotz is a professor of architecture, engineering, and business at the University of Virginia and the author of “Subtract: The Untapped Science of Less.” The idea of studying subtraction crystalized for Klotz when he and his son were trying to level a Lego bridge. By the time Klotz grabbed an extra Lego to even things out, his son had already solved the problem by removing one. Klotz now studies why we overlook subtracting as a way to improve things, including the various biological and cultural forces that push us towards more even when less would serve us better.