Previous chapters have outlined the significance of grandparent–grandchild relationships. If we consider other intergenerational relationships in addition to those between grandparents, parents and grandchildren, the obvious focus is the relationships between aunts/uncles and nieces/nephews. Aunts/uncles share approximately the same amount of genes with their nieces/nephews as grandparents share with their grandchildren (i.e., the average number of shared genes between aunts/uncles and nieces/nephews is 25 per cent) (see Figure 8.1). Thus, from the inclusive fitness perspective (Hamilton, 1964), the reasons why aunts/uncles choose to invest in their nieces/nephews are similar to the reasons why grandparents choose to invest in their grandchildren. Average relatedness to nieces/nephews who are monozygotic twins is even higher, and it is actually the same as relatedness to ones’ own children (50 per cent), whereas the average number of shared genes with nieces/nephews from half-siblings is 12.5 per cent (see Figures 8.2 and 8.3).