Mock discusses what scientists have discovered about this side of family dynamics in the natural world. Much of the biology of family behaviour rests on a simple mathematical relationship called Hamilton's rule, which links the benefits and costs of seemingly altruistic or selfish behaviour to the degree of relatedness between individuals. Blending natural history and theoretical biology, Mock shows how Hamilton's rule illuminates the study of family strife by throwing a spotlight on the two powerful forces - cooperation and competition - that shape all interaction in the family arena.