Steven Pinker -- How the Mind Works - Hampshire High Italian ...

452 J HOW THE MIND WORKSnot. As close kin, they feel a big extra dose of affection and solidarity. Butthough they share fifty percent of their genes with each other, each siblingshares one hundred percent of its genes with itself, so brotherly orsisterly love has its limits. Being offspring of the same parents, siblingsare rivals for their parents' investment, from weaning to the reading ofthe will. And though genetic overlap makes a pair of siblings naturalallies, it also makes them unnatural parents, and that genetic alchemytempers their sexual feelings.If people gave birth to a single litter of interchangeable w-tuplets,parent-offspring conflict would be a raw struggle among the siblings,each demanding more than its share. But all children are different, iffor no other reason than that they are born at different times. Parentsmay not want to invest one nth of their energy in each of their n children,but may, like shrewd portfolio managers, try to pick winners andlosers and invest accordingly. The investment decisions are not consciousforecasts of the number of grandchildren expected from eachchild, but emotional responses that were tuned by natural selection tohave outcomes that maximized that number in the environment inwhich we evolved. Though enlightened parents try mightily never toplay favorites, they don't always succeed. In one study, fully two-thirdsof British and American mothers confessed to loving one of their childrenmore.How do parents make Sophie's Choice and sacrifice a child when circumstancesdemand it? Evolutionary theory predicts that the main criterionshould be age. Childhood is a minefield, and the older a child gets,the luckier a parent is to have it alive and the more irreplaceable thechild is as an expected source of grandchildren, right up until sexualmaturity. (From then on, the reproductive years begin to be used up andthe child's expected number of offspring declines.) For example, theactuarial tables show that a four-year-old in a foraging society will, onaverage, give a parent 1.4 times as many grandchildren as a newborn, aneight-year-old 1.5 times as many, and a twelve-year-old 1.7 times asmany. So if parents already have a child when an infant arrives and cannotfeed them both, they should sacrifice the infant. In no human societydo parents sacrifice an older child when a younger one is born. In oursociety, the chance that a parent will kill a child drops steadily with thechild's age, especially during the vulnerable first year. When parents areasked to imagine the loss of a child, they say they would grieve more forolder children, up until the teenage years. The rise and fall of anticipated