The psychology professor is smiling. It’s a warm day in Cambridge, the first in a while, and the sun coming in through the window shines off his endearingly bald head. “The Hair of the Man,” a poem celebrating the joys of baldness, decorates the wooden door to his corner office on the 14th floor of William James. Entering his office is not unlike asking him a question—the result is a stream of new ideas and unexpected discoveries. On the giant bulletin board, strands of jewel-toned Mardi Gras beads dangle over journal articles and newspaper clippings. Tucked among his framed photographs is a picture from his meeting with the Dalai Lama. Rows of uneven books, different sizes and colors, line his shelves, but many are translations of the same one: his New York Times-bestseller, “Stumbling on Happiness.”