Warning: this episode contains mentions of suicide. A recently released study from researchers at Boston University examined the brains of 152 contact-sport athletes who died before turning 30. They found that more than 40 percent of them had chronic traumatic encephalopathy, or C.T.E., the degenerative brain disease associated with repeated hits to the head. Most of those athletes played football, and most played no higher than the high school or college level. John Branch, domestic correspondent for The New York Times, spoke to the families of five of these athletes.