Bennington College Professor Allen Shawn’s twin sister, Mary, was born autistic. When they were very young, their parents sent Mary away to get advanced care that they could not provide at home. As a result, Shawn felt a profound void in his life. That void, as well as the affect of Mary’s absence on his family, are the subjects of Shawn’s new memoir, Twin. He speaks with VPR’s Peter Biello about it.