A large study offers more evidence of a link between traumatic brain injuries and dementia later in life, with repeated injuries and severe ones posing the greatest danger. Researchers analyzed 36 years of health records of 2.8 million people in Denmark, where a national health system makes it possible to explore connections in a far-reaching way. ___21____ the risk was small. About 95 percent of people who suffered a brain injury never developed dementia. But a single severe brain injury increased the risk of later dementia by 35 percent ___22___ a person who never had brain trauma. A mild brain injury increased the risk by 17 percent. Each additional brain injury added to the danger. Scientists know that a blow to the head can damage brain cells, but they don't know exactly ___23___that might lead to later cognitive problems. This kind of study can't prove a cause-and-effect relationship, but researchers tried to eliminate the possible effects of age, gender, marital status and health, including depression. In a commentary in the journal, Dr. Carol Brayne of University of Cambridge's medical school in England wrote that improvements in care mean more people are surviving brain injuries, making it crucial to understand more about their long-term effects.