I'm with you, but I think you mean "I don't think there is anything inherently more painfull/brutal/dangerous about 15 rounds instead of 12 rounds" But I get the point and I agree. Boxing is kinda like smoking. The more you smoke, the greater the risk. And yes it's cumulative, from the beginning. But there still is precious little convincing evidence that rounds 12-15 in world title fights between qualified competitors was a significant risk factor, compared to other rounds or other factors.
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