“One compelling explanation for the elders’ greater contentment comes from the psychologist Laura L. Carstensen, founding director of the Stanford Center on Longevity. Her hypothesis, which she gave the wonky name “socioemotional selectivity,” is that older people, knowing they face a limited time in front of them, focus their energies on things that give them pleasure in the moment, whereas young people, with long horizons, seek out new experiences or knowledge that may or may not pay off down the line. Young people fret about the things they don’t have and might need later; old people winnow the things they have to the few they most enjoy. Young people kiss frogs hoping they’ll turn into princes. Old people kiss their grandchildren. “It’s hard to get an eighty-five-year-old to take inorganic chemistry,” Carstensen said. Maybe old people live literally like there’s no tomorrow.”