In The Better Angels of Our Nature, Steven Pinker convincingly makes the point that by pretty much every measure you can think of, violence has declined on our planet over the long term. More generally, “the world continues to improve in just about every way.” He’s right, but there are two important caveats.

One, he is talking about the long term. The trend lines are uniformly positive across the centuries and mostly positive across the decades, but go up and down year to year. While this is an important development for our species, most of us care about changes year to year—and we can’t make any predictions about whether this year will be better or worse than last year in any individual measurement…