League of Denial: The NFL, Concussions and the Battle for Truth by Mark Fainaru-Wada and Steve Fainaru:
The survey went out in 2001. The response was overwhelming, an indication of how badly the players wanted to be heard. Guskiewicz sent out surveys to all 3,683 living members of the NFL Retired Players Association; 2,552—over 69 percent—sent it back. The survey highlighted more completely what the original had only hinted at: More than 60 percent of the players reported sustaining at least one concussion during their careers; nearly a quarter had had at least three. More than half said they’d lost consciousness on the field or experienced memory loss at least once. But perhaps the most disturbing finding was the apparent correlation between the number of concussions and depression. Players who reported concussions were three times as likely to report that they were depressed. Guskiewicz wasn’t immediately sure why this was so, but he theorized that the concussions or the symptoms of the concussions—perpetual headaches, memory loss, erratic moods—were causing it.