For Malcolm and Simone Collins, declining birth rates across many developed countries are an existential threat. The solution is to have “tons of kids,” and to use a hyperrational, data-driven approach to guide everything from genetic selection to baby names and day-to-day parenting. They don’t heat their Pennsylvania home in winter, because heating is a “pointless indulgence”. Their children wear iPads around their necks. And a Guardian journalist witnessed Malcolm strike their two-year-old across the face for misbehaviour, a parenting style they apparently developed based on watching “tigers in the wild”. The Collinses are leading spokespeople for a movement called...