Kinship ties-the close relationships found within the family-have been a central focus of evolutionary biological analyses of social behavior ever since biologist William Hamilton extended the concept of Darwinian fitness to include an individual's actions benefiting not only his own offspring, but also collateral kin. Evolutionary biologists consider organisms not only reproductive strategists, but also nepotistic strategists. If a person's genes are just as likely to be reproduced in her sister as in her daughter, then we should expect the evolution of sororal investment in the same way as...