League of Denial: The NFL, Concussions and the Battle for Truth by Mark Fainaru-Wada and Steve Fainaru:
Nine months later came yet another NFL study in Neurosurgery. This one dealt with repeat concussions. Numerous previous studies had shown that one concussion left the brain vulnerable to another concussion if the brain wasn’t given time to heal. Guskiewicz had taken it a step further: Repeat concussions, he’d found appeared to increase the probability of dementia later in life greatly. But that wasn’t a problem in the NFL, according to Pellman et al. The league looked at how quickly players went back on the field and concluded that they were at no greater risk than if they had never been concussed at all. The logic was that because players returned to the field so quickly, they must have been okay or the medical staff wouldn’t have cleared them. This flew in the face not only of previous research but of widely known realities on an NFL sideline. First, players often didn’t report their injuries. Second, they hid their symptoms whenever they could. Third, NFL doctors often deferred to the wishes of coaches and players, just as Pellman had deferred to Parcells. As Steelers doctor Tony Yates had said: “Only a head coach can pull a player off.” The entire NFL culture was incentivized toward risk.
For the first time, the NFL also took on the issue of football and brain damage, a growing concern among researchers. The league’s scientific opinion? This wasn’t a problem in the NFL either. Boxers got brain damage. Football players didn’t. It was as simple as that. “This injury has not been observed in professional football,” Pellman and his colleagues wrote.