For this latest study, he and co-author Michael Morgan, a medical student, conducted three experiments. First, they analyzed what happened when men, aged from 22 to 50, hit a punching bag as hard as they could. The peak stress delivered to the bag - the force per area - was 1.7 to 3 times greater with a fist strike compared with a slap.
"Because you have higher pressure when hitting with a fist, you are more likely to cause injury to tissue, bones, teeth, eyes and the jaw," Carrier said.
The second and third experiments determined that buttressing provided by the human fist increases the stiffness of the knuckle joint fourfold. It also doubles the ability of the fingers to transmit punching force, mainly due to the force transferred from the fingers to the thumb when the fist is clenched.
In terms of the size and shape of hand anatomy, the scientists point out that humans could have evolved manual dexterity with longer thumbs, but without the fingers and palms getting shorter.
Gorilla hands are closer in proportion to human hands than are other apes' hands, but they and no other ape - aside from us - hits with a clenched fist.