If you spent 2018 mainlining misery about global warming, inequality, toxic politics or other anxieties, I’m here to break your addiction with some good news: The world got better last year, and it is going to get even better this year.

Poverty around the world is plummeting; half the world is now middle class; and illiteracy, disease and deadly violence are receding. These things don’t make headlines because they are gradual, relentless and unsurprising. That is why they are worth highlighting.The problems the world faces...