When Jerod Mayo announced his retirement at the age of 29 last month, it was surprising. But statistically, the surprise was in how long he made it. Mayo’s seven seasons were more than twice as long as the average linebacker’s career, and according to new research, NFL players’ careers are significantly shorter than they were just a few years ago.

Using data from Pro Football Reference, the Wall Street Journal has run the numbers and found that as of 2014, the average NFL career is 2.66 years. As recently as 2008, that number was 4.99 years.

The drops are more significant in some positions than others—nowhere is it more precipitous than at quarterback:

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The WSJ article doesn’t attempt a hypothesis, which is probably prudent—any drop this dramatic has to be the result of multiple factors. To say it’s just “the game has become more dangerous”—or its corollary, “players are more concerned about their safety”—is tempting but insufficient.

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My own pet theory is the rise of specialization in the NFL. With rosters as big as they are, and teams giving more weight to analytical methods of determining roster composition, player evaluation, and playing time decisions, I believe there are more NFL players than ever before who are good at one or two specific things, and deployed just for that. This specialization also makes them more replaceable. If you’ve got a guy who isn’t a complete player, and isn’t asked to be one, his particular skills may not actually benefit from experience. If those skills are purely physical, a 26-year-old simply isn’t going to be as fast or as reflexive as a 23-year-old. Especially in a league where salary cap math matters as much as anything, teams will happily plug in a guy on his rookie deal rather than pay to retain the guy with the expiring contract. The NFL has always been a revolving door; the turnover’s just faster now.