"China Chins"

That's an interesting theory. I suppose it could be a factor, although it's impossible to test. Does the fighter who gets knocked out get knocked out again because the earlier knockout weakened his punch resistance, or because he never had much to begin with??

Impossible to answer empirically I would say.

 

yea, a chin can get "cracked". best example is vargas.

took flush shots from quartey, laughs them off.

then gets "cracked" by tito, and suddenly he gets almost ko´d by wilfredo rivera soon after.

thats why most boxing careers are short.

 

I'm not arguing against this theory, and in fact agree with you that it's probably a contributing factor. Certainly, you would say, that along with the aging process itself, it accounts for why guys with rock solid chins in their primes, like Ray Mercer, sometimes have noticeably less sturdy chins when they get older. In these sorts of instances I think it is probably a MAJOR factor.

With a fighter who had minimal punch resistance to begin with, it is probably not as big a factor - although one would think that it does have an influence. Usually, however, the guys with the crappiest chins probably don't hang around long enough for it to come into play as much as with a guy like Mercer.

 

A fighters chin usually indeed declines with age. Mercer is a good example although he still took a tremendous beating from Klitschko without going down for the count and more or less quit vs Briggs.

I think the best example that satisfies your hypothesis is Roy Jones. He didn't show any chin problems whatsoever untill he got caught with what you could call a "lucky punch". I don't really believe in a lucky punch (a punch is thrown with that intent), but it landed flush nethertheless and knocked him out. After that he was never the same again.
Could also be mental.

A guy like Ali kept an excellent chin despite taking huge beatings. There are many other examples, Foreman and Holmes for instance.

 

I think it has a lot to do with the mental state of the fighter, not just the physical state. When someone's chin gets "cracked", in their minds, they start questioning their ability to take a punch and thus, become less relaxed in the ring. This leads to a downward spiral of losing confidence and getting hurt more and more by lighter punchers.

One of the ways to tell that a young fighter will have a good chin/bad chin is to just look at how relaxed they look while fighting and especially, while exchanging punches. Guys with titanium chins almost always have one thing in common, they look extremely calm and comfortable at all times while fighting.

 

Corrales' chin had never been that good, but after he got cracked by Castillo I, his chin got even worse.

 

It can also work the other way around, like in Foreman´s case.