The only way to “win” at the game of evolution is to leave surviving copies of your genes. Yet not just your children carry copies of your genes. Your siblings are just as closely related to you (50 percent shared genes) as your children; your nephews and nieces share a quarter of your genes, and your cousins one eighth. In a strictly Darwinian calculation, whatever cost you would bear to save one of your children you should be willing to pay to save two nieces or four cousins.5
