I have found that even jammers show more than a racing brief from personal experience. I know cyclists don't wear anything under their shorts because the extra seams will cut into their skin. Is that a reason why wrestlers don't wear anything under their singlets?

Jammers...
 

Agree with Shaulis on Jammers. I think the big difference is that the racing briefs keep everything in one place and there is little room for movement. Jammers, on the other hand, don't do that. Your stuff can go anywhere, even down the leg, which looks very obvious. The other drawback to Jammers is that when that happens, the only way to fix it is to reach down inside and adjust. With a brief its a quick hand movement to put all back in place.

When I wrestled (in the 70's) we were to wear a jock strap. Years later when I coached, the jock went out of favor and the guys just free balled. Never seemed to bother them with out the jock. Personally I like everything confined and held in place.

Speaking of "adjusting". Once I saw a football game (on TV) where the guy who was going to do the opening kickoff was real nervous and continually adjusted his crotch. For some reason the kick off got delayed and delayed and delayed, but the cameras couldn't go off him because he was going to do the kick off. Millions of viewers were treated to the sight of that guy's hand fondling his crotch.