According to a widely accepted model for happiness across our lifespan, people’s happiness rises early on, reaching a peak around age 30. It then declines in midlife before rising again. The model’s been around since 2008, with supporting evidence from studies conducted throughout the globe since the 1970s. But researchers have been witnessing a troubling shift that could upend this model. They’re seeing rising unhappiness for 18-25 year-olds in six English-speaking countries. The World’s Sarah Birnbaum reports.