I'll give my thoughts...

A boxer is used to hitting with his hands wrapped, foam padded and then padded by gloves, somtimes up to 16oz during bag work etc.

This is primarily to proect his hands because generally bones in your hand aren't large enough to disperse the force generated... if you hit a reletavily padded object such as a head guard you'll know about it if you do it enough times and you'll know about it... anybody who's competed here even WEARING gloves will probably have had to stick there fists in ice after a fight if they can bang even a little bit. That's from hitting a human skull, harder than concrete... but you can pretty much punch as hard as you want to all night long without SERIOUS pain.

Try that shit bare knuckle and you'll probably break your hand if it's a solid hit.

MMA fighters on the other hand (excuse the pun) fight and train with a relativly small ammount of padding on the hands, to allow for grappling and because 'striking' is not a major part of the sport.

because of being conditioned this way I believe that in a street fight an MMA fighter is neurologically wired to pull his punches a little more because his body knows that it will break it's bones if it doesn't.

A boxer doesn't have this neurological brake, so he's more likely to swing with everything bare knuckle at what is essentiall concrete. When you factor in the fact that the boxer works exclusively on developing his punching as his form of attack and hasn't developed any inhibitions to punching a target with full force:

Who is more likely to hit harder? A Boxer
Who is less likely to fracture their had? An MMA fighter.

nothing to do with strength, technique, stronger bones or what ever you want to say... MMA fighters have to exercise more restraint in punching more for various reasons so are more likely to do it when the risk of breaking their hand is highest.