A new, actual scientific study proves Santa has his limits. And they’re not the laws of physics. Every year on Christmas Eve, St. Nicholas delivers gifts to hundreds of millions of children around the world. But the real question isn’t how Santa manages to visit so many houses in one night, but why he consistently fails to visit so many children. John Park, a physician now studying public health at Harvard University, decided to find out which kids Santa does and doesn’t visit each Christmas. In a large-scale observational study, Park decided to map out Santa visits to the UK in 2015, focusing on the children who needed holiday cheer the most: kids in hospitals. His study was inspired by a childhood bout of sickness that left him hospitalized on Christmas (as he recounts in the video above). He got to experience the thrill of a visit from Santa—but he now knows many other ill children are not so lucky. Park and a few colleagues called nearly 200 hospitals with pediatric wards around the UK, asking staff who worked last Christmas whether anyone in the garb of Santa Claus had made a visit. Then they did a statistical analyses of which factors may have influenced his appearance. The first thing they discovered is that Santa didn’t appear to care which kids were naughty or nice—there was no correlation between his visits and nice-ness (measured by youth conviction and school absence rates for the area of each hospital). Then they measured whether distance from the North Pole made a difference. It didn’t. Finally, they compared Santa visitations with the deprivation rating (accounting for factors like income, employment, and barrier to services) of the area where each hospital was located. There, they found a clear correlation: the children in the most deprived regions were the least likely to have gotten a visit from Santa. That underprivileged children receive less attention on the holidays is hardly a novel idea. But when it comes to children in hospitals, it becomes more than a question of socioeconomic privilege—it’s a matter of public health. Even if the health value of holiday cheer is difficult to fully quantify. “It’s difficult to say whether a visit from Santa directly relates to improved health care benefits—probably not,” Park says. “But there certainly are elements whereby improving the environment in which care is provided certainly produces positive results in terms of health outcomes as a whole.” Park says he hopes the study will point out the need to address these kinds of social disparities, whether through policy changes or through increased local volunteering, so that every child has equal opportunity to get well on Christmas.