A 2000 study surveyed 1,090 former N.F.L. players and found more than 60 percent had suffered at least one concussion in their careers and 26 percent had had three or more. Those who had had concussions reported more problems with memory, concentration, speech impediments, headaches and other neurological problems than those who had not, the survey found.
A 2007 study conducted by the University of North Carolina's Center for the Study of Retired Athletes found that of the 595 retired N.F.L. players who recalled sustaining three or more concussions on the football field, 20.2 percent said they had been found to have depression. That is three times the rate of players who have not sustained concussions.
As scrutiny of brain injuries in football players has escalated in the past few years, with prominent professionals reporting cognitive problems and academic studies supporting a link more generally, the N.F.L. and its medical committee on concussions have steadfastly denied the existence of reliable data on the issue.
But in September 2009, a study commissioned by the N.F.L. reported that Alzheimer's disease or similar memory-related diseases appear to have been diagnosed in the league's former players vastly more often than in the national population — including a rate of 19 times the normal rate for men ages 30 through 49.
The study, which was conducted by the University of Michigan's Institute for Social Research, has not been peer-reviewed, but the findings fall into step with several recent independent studies regarding N.F.L. players and the effects of their occupational head injuries.