In the 19th and 20th centuries nostalgia was variously classified as an “immigrant psychosis,” a form of “melancholia” and a “mentally repressive compulsive disorder” among other pathologies. But when Dr. Sedikides, Tim Wildschut and other psychologists at Southampton began studying nostalgia, they found it to be common around the world, including in children as young as 7 (who look back fondly on birthdays and vacations).