If you live in a big city, you may have noticed new buildings popping up — a high-rise here, a skyscraper there. The concrete jungles that we've built over the past century have allowed millions of us to live in close proximity, and modern economies to flourish. But what have we given up by moving away from the forest environments in which humans first evolved? This week, we discuss this topic with psychologist Ming Kuo, who has studied the effects of nature for more than 30 years.