New research shows that being surrounded by tall buildings produces a “substantial” negative impact on mood. City dwellers have a 40 percent increased risk of depression and double the rate of schizophrenia. That’s the problem when cities are designing ever denser centers to accommodate for the worldwide trend of urbanization. Communities flourish with 30 to 50 homes per hectare (about 2.5 acres). That doesn’t work in cities. But there may be a sweet spot: Cities where buildings don’t rise above eight floors or so. Barcelona or Vienna show the future of healthy urban living.