A new study released Tuesday from one of the foremost neuroscience researchers in the world suggests children exposed to tackle football before the age of 12 are two to three times more likely to develop emotional and/or behavioral issues later in life, compared with individuals who began playing the sport at 12 years or older.

The study, published in Nature’s Translational Psychiatry, is the latest from Boston University neuroscience expert Robert Stern, and a follow up to previous research Stern and his colleagues published in 2015.

The sticking point of the study is its inclusion of amateur test subjects, meaning former American football players who only played up to the college level, not professionally in the NFL. As the authors note in the paper, this is believed to be the first study to show a relationship between early exposure to football and clinical dysfunction in a cohort that included both amateur and professional football players.

"I really wish I could say I was surprised" by the results, Stern told ESPN in an interview. "Instead, it was more, 'Oh yeah, this really is a big deal.' And it's just one more piece of the puzzle that, at least when it comes to youth football, has now gotten me over the edge to say, 'You know, we shouldn't be having our kids hitting their heads over and over and over while their brains are developing this way.’”

The researchers chose age 12 as an empirically based cutoff due to previous literature proving the male brain undergoes a key period of maturation between the ages of 9 and 12—in terms of gray and white matter volume, synaptic and neurotransmitter densities and other important neurodevelopmental milestones. It is estimated that during this time of peak neurodevelopment, current youth American football players experience a median of 240 to 252 head impacts in one single season.

The 214 former American football players who volunteered for the study took telephone and online cognition tests. The BTACT is a 20-min telephone-based objective assessment of cognition; the BRIEF-A is an online self-report instrument that measures executive function behaviors; the CES-D is an online self-report checklist of depressive symptoms; and the AES is an online measure of cognitive, behavior and emotional symptoms of apathy.

Results indicated those who began to play football prior to the age of 12 exhibited worse scores on the BRIEF-A, CES-D and AES, but not on the BTACT.

Still, researchers concluded from all the data available that men who began playing football before age 12 had twice the increased odds for clinically meaningful impairments in reported behavioral regulation, apathy and executive function, and three times increased odds for clinically elevated depression scores, compared with those who began playing at 12 or older.

“Age of first exposure to football may contribute to why some former American football players develop long-term clinical impairments, whereas others appear more resistant. Some support for this claim can be found in the setting of acute concussion, where children and adolescents are more vulnerable (compared with adults) to prolonged symptoms, disruptions in educational and social development and lower intellect and academic achievement,” the paper reads.

While this was not a CTE study, the authors did acknowledge that a young age of first exposure to football could be a risk factor or modifier of the neurological disease, and said they are currently conducting clinicopathological investigations in autopsy-confirmed cases of CTE to examine this possibility.

The authors acknowledge multiple limitations. The study participants comprised a convenience sample of volunteered, rendering randomization no longer possible. Since the research was conducted through online and telephone tests, it is not as robust as if it was done in person. Additionally, there was a wide age range in the sample and older subjects may have had fewer opportunities to play organized young football, and protective headgear could have varied widely across the group.

This newest study by Stern enters the public spat between the researcher, BU and the NFL. In 2012, the NFL agreed to donate tens of millions of dollars to concussion research overseen by NIH, which planned on Stern conducting the research at Boston University’s CTE center. After a falling out and some of the NFL money never going to Stern, a 2016 congressional study found that the NFL attempted to use its money as leverage to steer funding away from Stern—who had long been a critic of the league—and into the hands of a doctor with ties to the organization.

Then, in 2015, Stern published the precedent study to this current one. Three months later, several NFL-affiliated researchers published a paper in which they said they were unable to replicate Stern’s findings.

In the current Translational Psychiatry paper, Stern et. all specifically referenced that study that rebuked theirs.

“A recent study funded by the NFL examined the relationship between years of youth football participation and neuropsychological, neurological and neuroradiological outcomes in 45 former NFL players. That study was limited because a sizable subset of the participants did not play youth football, or only played 1 year. The authors argued that the study by Stamm et al. finding significant effects between AFE [age of first exposure] to football and cognition in former NFL players was limited by small sample size, decreased generalizability to all football players, arbitrary dichotomization of AFE before and after age 12 and lack of control for a history of learning disability. The present study addresses all of these concerns by examining AFE to football as a continuous variable in 214 former American football players, and continues to find a robust relationship between AFE and long-term clinical dysfunction.”