Before and after the season, the players also underwent elaborate brain imaging. Diffusion tensor imaging is a type of MRI that’s used to identify tiny changes in the structure of white matter (the neurons in the brain that are coated in myelin). The image measures fractional anisotropy (FA) of the movement of water molecules along axons. In healthy white matter, the direction of water movement tends to be uniform, but FA values decrease as movement becomes less ordered, and that process has been associated with head trauma. And in this case, the images of the boys’ brains at the end of the season showed a significant relationship between head impact and decreased FA in white-matter tracts. Boys who experienced more head impact had more changes.

“These decreases in FA caught our attention,” Whitlow said in a press statement, “because similar changes in FA have been reported in the setting of mild traumatic brain injury.”

“It’s difficult to say what the changes mean,” co-researcher Joel Stitzel told me, “but they do seem to be directly correlated with the impact that kids are sustaining.”

This was, again, among boys who had no signs or symptoms of concussions.

“The numbers here are pretty staggering. You have fewer than 2,000 people playing in the NFL, which gets all the media attention,” Stitzel told me. “But there's actually about 2,000 kids playing for every NFL player—3.5 million kids playing youth football in the U.S. About whom there is very, very little information.”

Last year researchers at Boston’s Brigham and Women’s Hospital found that NFL players who had begun playing football before age 12 had a higher risk of altered brain development, as compared to players who started later. And this August, Ann McKee, director of the Boston University Chronic Traumatic Encephalopathy Center, said that kids under 14 shouldn’t play football, reminding The Washington Post that kids’ heads are “a larger part of their body, and their necks are not as strong as adults’ necks. So kids may be at a greater risk of head and brain injuries than adults.”

“Other studies have looked at people involved in contact sports and compared them to people not involved in contact sports, and found similar changes to what we found,” said Stitzel. “But this is one of the first that’s quantifying what the actual exposure is. And it’s one of the first to look at the youth population.”

I asked him if this means people should stop letting kids play football, and he said no, which was surprising. I would never let my kid play football, and I don’t even have a kid. Stitzel’s argument is that there’s not definitive proof that youth football is bad for most kids.

“We aren't out to destroy football, by any means,” he said. “This is the type of work that's going to save and help football going forward.”