Final answer:
The rare occurrence of blood relatives killing each other is explained through kin selection, a concept from evolutionary psychology, which suggests that altruistic behaviors towards relatives increase the survival of one's own genes, as relatives share many common genes. This genetic favoring of kin, coupled with social norms and taboos, reduces internal violence within families.
Step-by-step explanation:
According to evolutionary psychology, the rarity with which blood relatives kill each other can be explained by the concept of kin selection. Kin selection suggests that behaviors, including altruistic ones that raise the survival and reproductive success of genetic relatives, are favored by natural selection. This is because relatives share a significant number of genes, and by helping a relative, an individual indirectly ensures the continuation of their own genetic material. Richard Dawkins, in his 1976 book, 'The Selfish Gene', describes how genes that promote such behaviors can become more common within a population if they aid in the survival of related individuals carrying the same genes.
This genetic inclination towards helping kin would naturally lead to a decrease in lethal behaviors amongst family members. Furthermore, cultural norms such as the incest taboo, as described by anthropologist Claude Lévi-Strauss, reinforce the separation of societal roles within groups, delineating who can engage in procreation and who are considered family, which further discourages violence within family units.