Researchers looked at the brains of deceased individuals under age 30 who had played contact sports. Of 152 former athletes with a history of repetitive head injuries, 63 had CTE. Those with CTE were disproportionately former U.S. football players and were significantly more likely to have played professional football. However, it’s important to note that several well-done, long-term epidemiologic studies have found no difference in neurodegenerative diseases in high school football players compared to those who did not play football.