Football’s concussion crisis has been part of the NFL for almost two decades. But the pros aren’t the only ones reevaluating their relationship with the game. Now, studies are finding that parents of younger children are increasingly concerned about the long-term impacts of playing football.

A national survey from 2015 found that 25 percent of parents do not let their kids play contact sports due to fear of concussions, while an Aspen Institute report recently found that participation in tackle football declined by 12 percent among children ages 6 to 12 between 2016 and 2017.

The research into the risks of youth football is still coming into shape, and there’s disagreement about just how universal and severe the risks are. Some researchers think football is dangerous for everybody; others are finding evidence that some kids might be more predisposed to health consequences than others.

In the last two years, some researchers have shown that head hits in youth sports increase the risk of developing chronic traumatic encephalopathy, or CTE, an untreatable degenerative brain disease with symptoms ranging from memory loss to progressive dementia. Other studies have shown that the longer a person plays football, the higher the risk they have for developing symptoms associated with CTE. So, case closed, right?

No — football is not the only risk factor in developing symptoms of CTE. The same study that found an association between repetitive head impact and dementia in CTE also found that cardiovascular disease and dementia in CTE were correlated. And a separate study of some 10,000 people found no association between participation in contact sports and later cognitive decline or increase in symptoms of depression. “We don’t see this being a massive epidemic across a huge swath of the population,” said Adam Bohr, a researcher in integrative physiology at the University of Colorado Boulder and one of the co-authors of the study.

But that study isn’t bulletproof either, as it checked back in with former high school athletes in early adulthood — so it could still be too early to see cognitive decline.

And just to complicate things further, there is another study that tracked Wisconsin high school football players from the 1950s. Those former players, surveyed decades later in their 60s, did not have different cognitive or depression outcomes compared to nonplayers. Likewise, another recent survey of 35 retired NFL players over the age of 50 did not find a link between the number of concussions a player incurred and his cognitive decline.

It is difficult to parse the research, let alone conduct it: CTE can only be diagnosed post-mortem. Unless scientists are looking at the actual brains of former players, argue researchers such as Dr. Lee Goldstein of Boston University School of Medicine’s CTE Center, they are not getting a complete picture.

Goldstein argues the case against football and other high-impact sports is open and shut. “Association is very clear. Risk increases with greater exposure, so if you don’t want to have this disease or any risk of it, you shouldn’t do it,” Goldstein said of playing football at any level.

Goldstein and colleagues published a paper in the journal Brain in 2018 in which they found that mice subjected to different kinds of hits showed early signs of CTE, even without signs of concussion, implying subconcussive hits could also start the progression of the disease.

C. Munro Cullum, a neuropsychologist at the University of Texas Southwestern Medical Center, wants to understand what factors make some individuals more vulnerable to the negative outcomes of concussions. To do that, he and colleagues are partnering with youth football leagues in north Texas to collect data on concussions incurred during games. The project, called the Texas Sports Concussion Registry, aims to shed light on the connection between differences in how the sport is played and how that translates to concussion numbers and recovery.

Most people with a concussion get back to normal within days or weeks, but “there is this significant minority that may have ongoing symptoms,” Cullum said. Other researchers have found that a person’s prior history of anxiety may increase the risk of a prolonged concussion recovery, or a family history of migraines might make someone more susceptible to experience post-concussion headaches.

Others are analyzing how all hits, not just concussions, affect players. Elizabeth Davenport, a researcher with the UT Southwestern radiology department, and her colleagues outfitted helmets for 60 high school players to measure every hit in a game. They then separated players into high- and low-impact groups and conducted fMRI scans to see how the groups differed.

Those in the high-impact group saw an increased grey matter volume, which is not normal for that age group.

In the low-impact group of players — and in most normal teenage brains — grey matter volume is generally reduced or “pruned” throughout adolescence into the early 20s to make way for new synapses. “Your brain is forming more highways and less just general run-of-the-mill tissue,” Davenport said.

It may sound alarming that football players who receive more hits experience abnormal brain development, but Davenport cautions that researchers still don’t know what the changes mean or if they are long term. She would like to be able to know if it’s possible to draw the line at a certain number of head impacts before a player needs to recover. Similar measures already exist in baseball, for instance. Pitch counts help pitchers avoid rotator cuff injuries. Could football leagues eventually institute hit counts, after which players need to take a break? Would that break even allow the brain to heal?

But Goldstein doesn’t buy it. There are just too many factors involved in individual health to pinpoint a single safety threshold in hits.

“There’s not a universal threshold for the number of cigarettes, and if you cross that line then you have cancer. It doesn’t work like that,” he said.

Though parents are responding to that message — a study out of Arizona found a 17-point decrease in the percentage of parents willing to let their child play contact sports between 2019 and 2017 — football as an institution is not going anywhere. It’s unlikely to be sued or legislated out of existence, at any level. A judge recently dismissed a lawsuit against Pop Warner, the largest youth football organization in the country, brought by two families of deceased players who claimed the organization hid the risks of youth football. Because so many factors affect brain health, it’s hard to just pin something like a suicide, depression or erratic behavior on playing football.

So all the research leaves us more or less at this finding: Playing a sport where kids get hit over and over again affects their brain development. How much those brains are affected appears to depend on how long they play, how hard they play, and likely a multitude of other physiological, sociological and genetic factors that researchers are just beginning to analyze.

Some researchers say it’s too early in the concussion research to offer judgement calls on whether kids should play football.

“I would want more information, as a parent, to make a better-informed decision and that’s where these studies come into play,” Davenport said.

But in his work examining the brains of dead teenagers, Goldstein has seen enough.

“I don’t recommend it as a physician, I don’t recommend it as a father and I don’t recommend it as a scientist,” he said. “I can’t.”