Some scientists studying the relationship between contact sports and memory or mood problems later in life argue that cumulative exposure to hits that cause a snap of the head — not an athlete’s number of concussions — is the most important risk factor. That possibility is particularly worrisome in football, in which frequent “subconcussive” blows are unavoidable.

On Thursday, researchers based at Boston University reported the most rigorous evidence to date that overall exposure to contact in former high school and college football players could predict their likelihood of experiencing problems like depression, apathy or memory loss years later.

The finding, appearing in The Journal of Neurotrauma, is not conclusive, the authors wrote. Such mental problems can stem from a variety of factors in any long life.

Yet the paper represents researchers’ first attempt to precisely calculate cumulative lifetime exposure to contact in living players, experts said. Previous estimates had relied in part on former players’ memories of concussions, or number of years played. The new paper uses more objective measures, including data from helmet accelerometer studies, and provides a glimpse of where the debate over the risk of contact sports may next play out, the experts said.