In this sample of 214 former American football players, those who began playing football before age 12 had >2 × increased odds for clinically meaningful impairments in reported behavioral regulation, apathy and executive function, and >3 × increased odds for clinically elevated depression scores, compared with those who began playing at 12 or older. Effects were independent of age, education and duration of football play. Younger AFE to football, in general, corresponded with worse behavioral regulation, depression, apathy and executive function, as well as increased odds for clinical depression and apathy. To our knowledge, this study is the first to show a relationship between younger AFE to football and reported clinical dysfunction in a cohort that included both former amateur and professional football players. There was no difference in the effect of AFE by highest level of play. These findings validate and expand upon our previous work in a small, entirely distinct sample of former NFL players,44 and extend the influence of AFE to football on clinical function to former football players who only played through high school or college. Overall, this study provides further evidence that playing youth American football may have long-term clinical implications, including behavioral and mood impairments.

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,46 and the authors concluded there were no relationships. 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 to football and cognition in former NFL players44 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. Inclusion of history of learning disability status as a covariate in the models had minimal influence on the results. Notably, effects were diminished for odds of clinical impairment when AFE was treated as a continuous variable. This could be because the continuous scale assumes a constant effect across all ages; however, there may be a critical age (that is, age 12) where the effects are most robust. Age 12 (an empirically based cutoff) indeed had superior model fit with clinical measures relative to surrounding age cutoffs (that is, ages 11 and 13).

The specific mechanisms underlying the present findings are unknown. Between ages 9 and 12 is a time of peak maturation of gray and white matter volume, synaptic and neurotransmitter densities and glucose utilization, among other neurodevelopmental milestones.30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42 These changes are occurring to structures such as the hippocampus and amygdala,30, 41 where the neural circuitry modulates clinical functions, such as emotion regulation and behavior.63, 64, 65 Structures like the amygdala have robust connectivity with the ventromedial prefrontal cortex, a region associated with depression in children and young adults.66 During this time of peak neurodevelopment, current youth American football players have been estimated, using helmet accelerometry, to experience a median of 252 head impacts per season in one study67 and a mean of 240 in another.68 RHI exposure over a single season of youth football can result in alterations of the left inferior fronto-occipital fasciculus and right superior longitudinal fasciculus white matter tracts.43 Exposure to RHI, in general, is associated with acute and long-term structural, functional and molecular brain changes based on research in active12, 69, 70, 71, 72 and former3, 7, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22 amateur and/or professional football players. The effects of RHI exposure on the brain at a young age may disrupt neurodevelopment and increase vulnerability to the long-term neuropsychiatric and cognitive impairments associated with prolonged exposure to RHI,1, 2, 3, 4, 5, 6, 8, 9, 10, 11, 12, 48 aging or likely both. AFE 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,73, 74, 75, 76, 77, 78, 79, 80 disruptions in educational and social development76, 81 and lower intellect and academic achievement.73, 76

This is not a study of risk for CTE or of other neurodegenerative disease. CTE currently cannot be diagnosed during life and the presence of CTE in this sample is unknown. Neuropathological evidence of CTE has been documented in former American football players. The current study found that AFE to football was associated with increased odds for impairments in domains described in the literature as being part of the clinical manifestation of CTE,47 that is, executive dysfunction, behavioral dysregulation, depression and apathy. These clinical features are not specific to CTE. Young AFE to football may be a risk factor or modifier of the clinical and neuropathological course of CTE, and we are currently conducting clinicopathological investigations in autopsy-confirmed cases of CTE to examine this possibility.82

We found no association between AFE to football and cognition as measured by the BTACT. This finding was unexpected, given previous work from our group showed that former NFL players who began playing football before age 12 exhibited worse neuropsychological test performance on measures of episodic memory and executive function at mid-life, compared with those who began playing football at 12 or older.44 In the current sample, only 15 (7.0%) participants were clinically impaired on the BTACT, whereas rates of impairments were >40% across all other clinical measures. Higher rates of impairment on non-BTACT measures may be due to their self-report nature, particularly as this convenience sample of self-selected participants may have been more likely to participate because of perceived clinical symptoms. Alternatively, although the BTACT is valid, convenient and cost-effective, telephone assessment of cognition is not ideal and the BTACT is not a comprehensive assessment of cognition. The BTACT may not be sensitive to cognitive impairment in this relatively young sample, and the global score that was used in the present study may not capture the diverse and, at times, subtle deficits associated with RHI exposure. Furthermore, evaluation of neuropsychological and neuropsychiatric function through telephone and online instruments precludes the ability to behaviorally observe the participant, limiting opportunity for the evaluator to monitor concentration and engagement in testing, particularly in the context of symptoms of depression and apathy. That said, the largely normal BTACT scores in the present sample argue against potential confounding from lapses in attention or lack of engagement in testing. Future work should further investigate the relationship between AFE to football and cognition using comprehensive neuropsychological testing.

There are several additional limitations to the present findings. As previously mentioned, this is a convenience sample of self-selected participants and randomization of individuals to groups based on their AFE to football is not possible. A convenience sample could potentially lead to bias effects, especially if AFE plays a role in selection. The findings can only be generalized to male former football players, and the relationship between AFE to other contact sports (for example, soccer) and clinical outcomes, including female contact sports, is unknown and should be the target of future research. There was a wide age range in the sample and older subjects may have had fewer opportunities to participate in organized youth football because it was not widely available until the 1960s. The style of youth football play could have differed across the age groups of the sample, including differences in type and use of protective headgear. We addressed the concern for confound of differences in era of play by including age as a covariate. The causal relationship between AFE to football and long-term clinical outcomes remains unclear, partially because this study was cross-sectional. The cross-sectional study design also limits interpretation of the associations among the clinical outcome measures. The tests examined assess symptoms that often co-occur, with bidirectional relationships (for example, depressive symptoms can affect performance on cognitive tests, cognitive impairment can also lead to symptoms of depression, depressive symptoms and impaired cognition can both be clinical manifestations of a single underlying disorder). Although the mixed-effect and GEE models used accounted for intercorrelations among the clinical measures, it remains challenging to disentangle their exact interactions and subsequent influence on the current findings. In particular, it is unclear whether the reported symptoms are part of a single clinical syndrome, or reflect distinct pathophysiological processes, possibly from the effects of exposure to RHI on strategic brain regions that modulate each of the clinical functions examined. Importantly, although we found significant mean group differences in reported clinical function between the AFE groups, our results underscore that there is significant individual variability in this relationship (Figure 1). Findings from the current study should not be used to inform safety and/or policy decisions in regards to youth football. Any decisions regarding reducing or eliminating youth football must be made with the understanding of the important health and psychosocial benefits of participating in athletics and team sports during pre-adolescence. Future longitudinal studies that objectively monitor the clinical function of youth football players throughout life, including those who do not go on to play football at the high school, college or professional level, are ultimately needed to understand the long-term neurological safety implications of youth tackle football.