“It seems like such a trivial reason for murder. When God belittled Cain’s gift to him of produce from his own garden, then praised his brother Abel for offering a sheep, Cain snapped. But as you get ready to gather with your family and unwrap presents, the Bible’s first homicide starts to make sense.” Among evolutionary behaviorists studying the complicated sibling dynamic, the “prevailing theory is that it all comes down to math.”