(Reuters) – A Baltimore Ravens offensive lineman pursuing a doctorate in mathematics announced his retirement from professional football on Thursday, and team officials said a study linking NFL players to brain disease was a factor in his decision.

John Urschel, 26, a Ph.D. candidate at the prestigious Massachusetts Institute of Technology who has been dubbed the “smartest player in the National Football League,” called Ravens Coach John Harbaugh and said he was retiring, Harbaugh told a news conference.

“That was something that’s been on his mind for quite a while, throughout the off season,” Harbaugh said.

Urschel has not spoken to the media about his reasons for retiring after just three seasons in the NFL, all with the Ravens.

The Baltimore Sun cited team sources indicating his decision was related to a study released this week on chronic traumatic encephalopathy, or CTE, a debilitating brain disease.

According to research published in the Journal of the American Medical Association, the brains of 99 percent of former NFL players studied showed signs of the disease, linked to repeated hits to the head that can lead to aggression and dementia.

CTE is linked to the head-to-head hits that have long been a part of the sport, although the NFL and school leagues have been tweaking the game in recent years to limit blows to the head.

Urschel, a 6-foot, 3-inch (1.9 m), 300-pound (136 kg) player who joined the Ravens from Penn State University, is studying applied mathematics at MIT and was the co-author of a research paper titled “Spectral Bisection of Graphs and Connectedness,” that was published in 2014, the same year he began his pro career.

An avid chess player who reads math books to relax, Urschel was featured in a television commercial in which he explains the technology behind noise-cancelling headphones to NFL star J.J. Watt, who uses the technology to tune out the lecture.

In a 2015 article he wrote in The Players Tribune, Urschel said he loved playing football and accepted the risk of brain injury.

“Objectively, I shouldn’t. I have a bright career ahead of me in mathematics,” he wrote, adding that he was not seeking to get rich playing football.

“The things I love the most in this world (reading math, doing research, playing chess) are very, very inexpensive,” he wrote.