The Concussion Legacy Foundation states that “one concussion in the absence of other brain trauma has never been seen to cause CTE.”

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The Concussion Legacy Foundation adds that "the best available evidence tells us that CTE is caused by repetitive hits to the head sustained over a period of years. This doesn’t mean a handful of concussions; most people diagnosed with CTE suffered hundreds or thousands of head impacts over the course of many years playing contact sports or serving in the military."

According to the Boston University’s Chronic Traumatic Encephalopathy (CTE) Center, CTE is a “progressive degenerative disease of the brain found in people with a history of repetitive head impacts often incurred during contact sport play, military service, and other activities that involve repeated blows to the head.”

A subconcussive head impact is a “bump, blow, or jolt to the head that does not cause symptoms. This differs from concussions, which do cause symptoms. A collision while playing sports is one way a person can get a subconcussive head impact.”

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In soccer​, subconcussive head impacts can occur when players head the ball. According to a recent literature review of concussions in soccer, "studies are lacking but do not demonstrate intermediate or long-term adverse neurocognitive effects from heading the ball in soccer."

The Boston University CTE center states that “CTE is caused by repetitive brain trauma. This trauma includes both concussions that cause symptoms and subconcussive hits to the head that cause no symptoms. At this time the number or type of hits to the head needed to trigger degenerative changes of the brain is unknown.”

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The National Institutes of Health (NIH), the United States government agency which serves as the largest biomedical agency in the world, recently acknowledged that CTE “is caused in part by repeated traumatic brain injuries.”

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However, the Concussion in Sport Group, an influential group that shapes concussion policy in sports, “have consistently played down the connection between CTE and brain injuries sustained in sport.”