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The opening quotation comes from a discussion of Anglo-Saxon law. An individual could be liable to pay wergeld for the slaying of his mother’s kinsman by his father’s kinsman and be entitled to receive wergeld for the same slaying, because each individual combined two lines of descent. The individual is divisible. Just as his loyalties can be divided by obligations to the two sides of his family, so too can his genome be divided between genes he shares with his mother and genes he shares with his father. Blood is thicker than water, and blood does not mix (in the sense that genes do not blend).

Genetically determined behaviors that benefit the father’s side of the family may be favored by natural selection when a gene has been transmitted by a sperm but not when the same gene has been transmitted by an egg. Conversely, a behavior that benefits the mother’s side of the family may be favored when a gene has been transmitted by an egg but not when the same gene has been transmitted by a sperm. In such circumstances, imprinted alleles, genes that are differently expressed when inherited via eggs and via sperm, can supplant unimprinted alleles that are expressed independent of parental origin (Haig, 1997, 2000b).

Imprinted genes have been considered prime candidates for involvement in disorders of human social interaction, such as autism and schizophrenia, because of their predicted role in interactions among kin (Badcock and Crespi, 2006; Isles et al., 2006; Crespi, 2008; Goos and Ragsdale, 2008; Úbeda and Gardner, 2010, 2011). Not all social interactions promote imprinted gene expression, however. The principal purpose of this paper is to clarify the rather specific conditions that favor stable maintenance of imprinted gene expression, but this task requires a broader understanding of how humans innately categorize kin. These questions will be addressed with a particular focus on effects of partner change and on internal genetic conflicts during sexual maturation and adolescence.

ASYMMETRIES OF RELATEDNESS

Consider an imprinted locus at which the established allele is silent when paternally derived but expressed at level x >0 when maternally derived. This pattern of expression is an evolutionarily stable strategy (ESS) when two conditions are met:

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