Conservative commentators have been bemoaning the decline of the American man almost as long as the American man has been in existence. As it turns out, they are right: Men these days are a mere shadow of what we once were. We've become physically weaker than our ancestors. We're slower runners. We can't jump as high as we once did. As Peter McAllister, an anthropologist with the University of Western Australia and the author of the new book "Manthropology: The Science of Why the Modern Male Is Not the Man He Used to Be," puts it, we might be the "sorriest cohort of masculine Homo sapiens to ever walk the planet."