“Why are you architects not interested in people?” Ingrid Gehl asked her new husband, Jan. “What do you think about the fact that your architecture professors take their photos at four o’clock in the morning . . . without the distraction of people in the photos?”

In the early 1960s, and in many cases still today, these were forbidden questions, particularly among those we think of as designers–architects, city planners, and engineers. Then and now, designers consider human needs for health, survival, safety, and comfort through building codes and best practices. Psychological needs are only an afterthought–at best.

Ingrid, however, was no conventional designer–she was a psychologist. And by entertaining such questions, Ingrid and her husband took the first steps on a journey to create city spaces for the full range of human needs. The Danish couple’s ideas have since made life better in cities like New York, Moscow, Buenos Aires, Sydney, and London. Of course, many parts of many cities still seem optimized for buildings and cars. But the story of Ingrid and Jan is a model for what partnerships between behavioral scientists and designers can look like today.

In the early years of Ingrid and Jan’s collaboration, seeking answers to Ingrid’s questions, the new couple studied public spaces. They were among the first to do so systematically, recording details like the number of people sitting, standing, or walking in parks, streets, and squares. In one of their studies, they spent months observing old Italian cities, which they noticed “seethed with life.” One reason for the vitality of these old Italian cities, they theorized, was they “had not yet been reorganized by rational planners.” (The post-World War II trend towards “rationalist” approaches to city planning modeled cities as machines made up of buildings and roads.) As psychologists like Daniel Kahnemann and Amos Tversky exposed the limitations of mechanistic theories about human rationality, Ingrid and Jan suggested that similar flaws were holding back city design.

Jan and Ingrid shared insights from the first decade of their collaboration in two books, both published in 1971. Jan’s book was titled Life Between Buildings , and Ingrid’s was called Bo-miljø, which translates loosely as “Living Environment.” In Bo-miljø, Ingrid challenged readers to recognize the humans they were designing for. She emphasized that people need to stay healthy, comfortable, and safe, and psychologically well. And cities could help. They could meet our need for human contact, or for privacy. They could enhance how we experience life, how we share and create ideas, and how we walk, play, and stay active.

Ingrid’s perspective, which may seem obvious to behavioral scientists, should have unveiled a dangerous blind spot for architects, planners, and engineers. But Bo-miljø had limited impact, even in Denmark and even among the narrow audience of professionals for whom it was written (most of whom were not the same gender as Ingrid). Whatever the reasons, Ingrid’s book quickly went out of print and she went on to a long and distinguished career practicing child psychology, occasionally contributing to her old profession, informally, through Jan’s work.

Meanwhile, Jan’s Life Between Buildings became a planning classic and is now in its sixth edition. Through his life’s work, Jan developed the “Public Space Public Life” survey, which is used around the world to generate snapshots of city conditions, just as he and Ingrid had done in Italy. As a public intellectual, Jan has shared human-focused planning perspectives and methods with millions of people. And through his design firm, these ideas have transformed cities all over the world.