But for all the buzz given to the studies that uncover telltale genes, rarely mentioned are the most obvious and familiar examples of how genes aren’t destiny: identical twins. They share the same genome and, usually, the same parents, same neighborhood, and same food. And yet, as anybody who’s ever met a pair knows, they are not the same person. Why? “Ten years ago the prevailing theory was that there must be systematic differences in their environments,” says Eric Turkheimer, a professor of psychology at the University of Virginia. One twin is favored by her mother, s...