Whole-body scans show that males average 61 percent more muscle mass then females, with most of that difference in the upper body.30 Men’s extra brawn, moreover, is added during puberty, when testosterone levels shoot up, accelerating muscle growth in the arms, shoulders, and neck.31 In this regard, human men resemble male kangaroos, whose upper bodies also enlarge during adolescence to help them fight.32 Enhanced upper-body muscularity in male humans might also have been selected for hunting, but we cannot rule out aggression. The second fact is literally staring us in the face. Consider
