When it comes to sports with striking, I think the "glass jaw" term gets thrown around too much. When a fighter is stopped, and it's not because of exhaustion or type of submission(corner, dr., fighter), it is normally due to at least one blow of heavy impact. That blow is usually labeled as being a hard shot, or being landed on an opponent with a weak chin. I think the label is sometimes determined by the fighter's reputation, rather than reality. While I believe a certain amount of powered is needed to induce a knock out blow, I think more emphasis should be given to the blows accuracy. Accuracy may be the wrong word here, because many so call "landed" punches do not automatically result in a knock out. I think the strike has to penetrate in a way that I do not fully understand to produce the knock out. I think that shot has to somehow disrupt the flow of energy in the nervous system, and I don't know of any sure method to produce this effect 100% of the time. I do believe that there are varying degrees of punch resistance, but I think that luck and defense really weigh in here. Often, a fighter's defense is measured by the number of times an opponent clearly misses. Maybe the ability to avoid a debilitating blow should factor in. I partially arrive at this way of thinking by arguing with most often those with little boxing experience. I'll see a shot that is not flush, and one of these people will say something along the lines of "it touched him." I'll argue that a blow of such insignificance means little, often to no avail. Ray Mercer, George Chuvalo, Marvin Hagler, and Jake Lamotta all have a punch resistance that few possess. Most everyone else is use to avoiding such a blow entirely, or are good at avoiding the deciding impact it takes for being knocked out.
I think everyone is different. There is a point at which any fighter can be knocked down. This could be attributed to being relaxed, anxious, not seeing the punch, or landing it in the correct spot. A fighter labeled with a great chin can become more aware of these things as they progress in their careers. David Tua was knocked out in the amatuers, but hardly ever rocked as a pro. The shot he was knocked out was a shot he didnt see. Did he become more aware of punches coming his way by better concentration? I think so, and probably why you see some brutal knockouts as fighters become more fatigued. Evander Holyfield was not the biggest puncher, yet he was able to drop just about everyone. This is because he was one of the most accurate punchers ever. His ability to counter and land his shots right on the button is something rarely seen in any fighter. I think the "sweet spot" on a fighter gets larger as they age or sacrifice conditioning as well. You've seen fighters with decent chins in their prime become easier to knock down towards the end of their careers and guys with decent chins but poor conditioning get suprisingly stopped.
That's right. Most fighters chins are shaky to one degree or another. Not glass, necessarily, but not exactly granite either. Truly solid chins are, in fact, quite rare. And a good thing too - boxing would be less exciting if everybody could take shots like these guys.
It's more mental then any common fan wants to admit. A fighter who isn't relaxed is more or less going to have issues taking punches. Wladimir Klitschko & Amir Khan come to mind as current fighters who are so unbelievably tense/high strung when taking shots that I can't help but wonder how much better they would take them if they relaxed.
Punch resistance is the #1 attribute to have in Boxing. 99% of the time a fighter goes nowhere if he does not have a Chin.
Alll three, no. Patterson and Moorer were blow up light heavyweights. If Carlos Monzon fought Marciano and got crushed would we say he had a glass chin ? Shavers had a decent chin, not great but decent. He took Holmes best shots and never went down. He took some bombs from Roy Williams, Tex Cobb, Jeff Sims, ect.