When a boxer's ability to take a punch is reduced after a knockout loss, how much of it is physical, and how much of it is mental? A good example of what I'm talking about would be Roy Jones. After he got knocked out by Tarver in the second fight, his punch resistance took a nosedive. In that case, I would say it was a mix of both. It was physical because he was 35, which is old for a boxer, and the fact that he dropped down from HW to LHW, and the knockout obviously had some mental effect on him.
I think it is physical. The way I heard it described best was by Chuck Liddel. His theory was that when you get a guy who takes solid punches without any problem you take the punishment, but you are just ignoring it. Eventually you're body can't take the punishment without becoming seriously injured. However, you have 'trained' your brain to believe that you will keep going, even if you're taking huge punishment. So what happens is your brain shuts off your consciousness, at the first sign of a punch, to prevent you from taking any more punishment. I think of it more like a biological response to your conscious actions.
I think it's both. Look at Ricky Hatton for example, he would take punishment in his fights but he was able to shake it off. Eventually you could see it start to affect him, I first noticed in the Collazo fight and Collazo was not a big hitter at all. By the time he fought Mayweather his resistance wasn't anywhere near as good and, with both the accurate punching of Mayweather and Hatton throwing caution to the wind because he was so far behind, Mayweather was landing punishing blows. Then came the shot that put Hatton out. Then he fought Pac, it looked like every single shot hurt him. I think his mind was sub-consciously telling his body that he couldn't take it and he would shutdown. Then the big shot landed and it was game over. I don't think he could've been the same again after the Mayweather fight because he was mentally done, with that in mind I would say that the punishment he took with his style through the years was physically wearing him down too.
The only way you understand this is by being sincerely conscious... Following boxers as they age... It's not a simple answer...many factors
I like it. Physiologically, your body/brain loses tolerance for trauma with more trauma. Over any given fight, the brain is concussed sometimes dozens of times, and that explains why sometimes guys take big shots all night, then all of a sudden when a punch lands that doesn't appear any harder than he's taken so far, the guy's lights turn off. It was an accumulation of trauma. It happens like that over a career as well. Your brain decides it's had enough of punishment and turns off earlier as a self-preservation mechanism.
I would say it's a little bit of both. There is no question that age does a number on even the best of chins. Sure, we can all point to aging fighters like McCall or Hopkins who maintain their sturdy mandibles well past their primes, but more most fighters go in the direction Jones did - perhaps not as dramatically, but their chins do deteriorate. Take Ray Mercer and David Tua. In their primes, you'd be hard pressed to get either of these guys off their feet, but as they got older, they didn't take quite the shot they used to. Age does that. And, once an aura of invincibility is pierced, fighters tend to lose the mental confidence that went along with being able to take punches at a top level. Thus, I would say that while declining chins are, at least in their origin, probably largely a physical phenomenon, the mental factor plays a larger role once the chin has been shattered.