Jeff Lageman, a defensive end who played 10 seasons in the N.F.L., likened the phenomenon to a spat involving him and his older brother.

“He’d beat the hell out of me roughhousing and stuff, but if anybody tried to do the same thing, it was like, ‘Whoa, wait, you’re not allowed to do that; I’m his brother and you’re not,’ ” Lageman said. “It’s almost like the same mentality in football. You want to stand up for one of your own. And when you do that, guys do that because they feel obligated to do it.”

Carter characterized the N.F.L. locker room culture as “a different animal” compared with other popular team sports. While interviewing and surveying 104 players for his 2009 book, Carter said he was fascinated to discover what he considered a genuine lack of camaraderie. It was hardly a representative sample, he cautioned, but there were undercurrents that created a lack of trust and hindered the forming of strong relationships: rampant turnover; unceasing competition; nonguaranteed contracts; the sheer size of the roster, more than twice as large as in baseball or hockey.

“I think it’s part of that ‘We all have short careers, we’ve got to get what we want right now’ mentality, ‘We’ve got to protect ourselves,’ in that sense,” Carter said. “Maybe it comes off as protecting their teammates, which is what we would all like to think, that there’s this real social bond that exists among these guys, but maybe not so much as you’d think.”

In professional sports, the locker room is more than a place where athletes dress. It is where players culled from diverse backgrounds, and at different stages of their lives and careers, ostensibly strive to sacrifice individual goals for the good of the team. It is where ribbing and banter abound, and sometimes, as demonstrated with the Miami Dolphins’ bullying scandal that engulfed the league last season, it can turn ugly.

“There are things that would be said in the locker room, on the practice squad, on a football field, trying to get into that person’s head,” said Antonio Garay, a defensive lineman who played seven N.F.L. seasons. “Because there is a huge psychological part of football.”